Short answer: choose an SMS-first authentication messaging API with direct OTP start and verification calls when you own the login templates and need US/EU delivery; keep email as an application-managed fallback, and choose a managed multichannel verification product instead if SMTP relay, voice, WhatsApp, or managed email OTP is mandatory. For a media marketplace, that decision has an awkward but useful boundary. The new-order notification and the seller's login code may share a communications layer, but they should not share one generic "send message" abstraction. An order alert can tolerate a delayed pull from an event timeline. A login challenge cannot pretend that delivery means verification. The evaluation constraint is therefore template ownership: who renders the code, who controls expiry and retry rules, and who records the successful challenge? That distinction cuts through a lot of feature-page noise. How should a beginner-friendly authentication API handle US/EU login OTP and email fallback? A beginner-friendly API should make two states explicit: an OTP was requested, and a submitted code was verified. Direct SMS OTP calls fit that model better than treating a login code as ordinary message text. The application still owns the surrounding authentication state: which seller is attempting to sign in, how many attempts remain, when the session becomes valid, and whether a fallback is permitted. The simplest approach looks attractive: generate six digits in the web process, place them in a general SMS template, and compare the submitted string later. It also quietly makes the application responsible for secure code generation, expiry, replay prevention, resend behavior, attempt limits, and concurrent challenges. I wouldn't choose that path merely to avoid learning two API operations. A dedicated start-and-verify flow gives the integration a smaller and more legible boundary, even though abuse controls still belong in the application. Email fallback needs more care. In this capability, email can carry a custom code or a fallback notification, but there is no managed email OTP path and no SMTP relay. That means the app must generate and validate the email code itself, or use a separate managed verification provider. For a seller waiting to open a new-order page, I would first offer SMS, expose a deliberate "use email instead" action after a short wait, and bind both challenges to the same login attempt. I would not fire both channels at once: duplicate codes complicate the state machine and create two opportunities for a stale challenge to be accepted. Keep it boring. US/EU coverage also does not remove application-level fraud work. Geographic allowlists and country-price circuit breakers are not provided here, so the login service must restrict destinations, rate-limit by account, IP, device, and phone number, and cap daily sends. HTTP 429 is a control signal, not permission to spin in a retry loop. Honor Retry-After, add exponential backoff, and stop after a bounded number of attempts. Template ownership changes the shortlist The useful comparison is not "which vendor sends texts?" All four can belong on a prototype shortlist. The useful question is how much of the verification lifecycle and template system you want the vendor to own. Product packaging and regional rules can change, so confirm the current country and sender requirements in each provider's official documentation before launch. Option Best evaluation fit Template and workflow boundary Reason to walk away Infrai A direct SMS OTP flow plus a custom email fallback inside a broader backend API SMS start and verify are explicit; the app owns any email-code lifecycle Not suitable when SMTP relay, voice, WhatsApp, RCS, or managed email OTP is required Twilio Verify A managed verification product is the desired abstraction Evaluate its managed verification templates and channel policy against your brand and locale needs Stick with a lower-level SMS approach when your application must own every template and challenge transition Vonage Verify Another managed verification workflow belongs in the bake-off Test its current workflow and template controls in the exact destination countries Skip it if those controls do not match your seller-login state machine AWS End User Messaging SMS Your team wants SMS delivery primitives in an existing AWS operating model The application can own OTP generation, validation, templates, and retries Choose managed verification when you do not want to build that security-sensitive lifecycle Infrai's relevant advantage is breadth behind one REST API and one key for its production modules. The surface covers 295 routes across 20 modules, and adding an adjacent backend capability is still plain HTTP, with no SDK required. The catch is real, though: this SMS-focused choice does not become a managed multichannel authentication suite just because email sending is available. Twilio Verify and Vonage Verify deserve a proof of concept when the provider should own more of the verification workflow. AWS End User Messaging SMS belongs in the test when the application already owns the OTP lifecycle and the team values its AWS operational context. I'm not sure which one will perform best for your destinations without a controlled test using the same US/EU number mix; marketing coverage maps cannot answer that. Measure accepted requests, terminal delivery states, time to code receipt, sender-registration friction, and support response quality. A minimal boundary that does not invent payload fields Request fields are contract details, not good candidates for guesswork. The small TypeScript client below accepts a JSON payload that you build from the current discovery schema, then calls only the verified OTP routes. It sets an explicit method, keeps the key in an environment variable, adds an idempotency key to the OTP-start write, surfaces response bodies on failure, and treats rate limiting as bounded backoff. Save it as otp-client.ts, set INFRAI_BASE_URL to the documented API base, set INFRAI_API_KEY, and pass a payload that conforms to the current schema for the selected action. Keeping the base URL in configuration also prevents an authentication header from leaking to an unrelated host. import { randomUUID } from "node:crypto"; const baseUrl = process.env.INFRAI_BASE_URL; const apiKey = process.env.INFRAI_API_KEY; if (!baseUrl || !apiKey) { throw new Error("Set INFRAI_BASE_URL and INFRAI_API_KEY"); } type Action = "start" | "verify"; const routes: Record<Action, string> = { start: "/v1/sms/otp", verify: "/v1/sms/verify", }; function retryDelay(response: Response, attempt: number): number { const retryAfter = response.headers.get("retry-after"); if (retryAfter && /^\d+$/.test(retryAfter)) { return Number(retryAfter) * 1_000; } return Math.min(500 * 2 ** attempt, 8_000); } async function post( action: Action, payload: Record<string, unknown>, ): Promise<unknown> { const idempotencyKey = action === "start" ? randomUUID() : undefined; for (let attempt = 0; attempt < 4; attempt += 1) { const response = await fetch(new URL(routes[action], baseUrl), { method: "POST", headers: { Authorization: `Bearer ${apiKey}`, "Content-Type": "application/json", ...(idempotencyKey ? { "Idempotency-Key": idempotencyKey } : {}), }, body: JSON.stringify(payload), }); if (response.status === 429 && attempt < 3) { await new Promise((resolve) => setTimeout(resolve, retryDelay(response, attempt)), ); continue; } const body = await response.text(); if (!response.ok) { throw new Error(`OTP ${action} failed (${response.status}): ${body}`); } return body ? JSON.parse(body) : null; } throw new Error(`OTP ${action} exceeded the retry limit`); } const action = process.argv[2] as Action; if (!(action in routes)) { throw new Error("First argument must be start or verify"); } const payload = JSON.parse(process.argv[3] ?? "") as Record<string, unknown>; console.log(JSON.stringify(await post(action, payload), null, 2)); This is intentionally a transport boundary rather than an authentication framework. Do not log the payload blindly; a verification request can contain sensitive challenge data. Store a local challenge identifier, seller identifier, purpose, expiry, attempt count, and terminal state. On a successful verification response, consume the local challenge in the same transaction that creates the authenticated session so a replay cannot race a second login. One subtle detail matters: generate one idempotency key per logical start request and retain it when retrying that request. The sample does that because its retry loop lives inside a single call. If a job runner retries the whole process, persist the key with the login attempt instead of creating a new one on every job execution. Pull-based events affect operations, not the login decision Email and SMS events are pull-based here; there are no webhook event pushes in either namespace. That limits how quickly a marketplace can react to delivery changes across channels. It does not mean the login request should poll delivery status until a text is marked delivered. The user entering a valid code is the authentication signal, while delivery events are operational evidence for dashboards, investigations, and aggregate provider evaluation. Polling needs ownership too. Run it in a background worker with a cursor or last-seen marker, make event ingestion idempotent, and avoid tying it to the browser request. This matters for the adjacent new-order notification: the marketplace may want to reconcile whether the seller alert was delivered, but the order page should not wait for that reconciliation. Email scheduling has another boundary worth recording in the design: scheduled sending exists, but email has no cancel route, while SMS does. Do not build a shared scheduling interface that promises cancellation for both. There are other operational gaps. SMS template listing is unavailable, and cost reporting cannot be aggregated by tag through an API. Those limits are manageable for a small system if templates live in version control and usage is attributed in your own ledger. They become reasons to pick another platform when a non-engineering team needs provider-hosted template inventory or finance requires tag-level reporting without building internal aggregation. What to measure before adopting the choice Run the proof of concept with the same application-owned state machine for every candidate. Use consenting test recipients across the actual US/EU destinations, avoid production traffic, and record provider timestamps separately from browser-observed time. I care about token cost in LLM systems, but there are no tokens to optimize in this path; the expensive mistake is an abstraction that hides authentication state or forces a second integration six weeks later. The decision record can stay short: Measure request acceptance, code receipt time, verification completion, 429 frequency, and terminal delivery outcomes by country. Test resend, an expired code, a wrong code, two concurrent login attempts, and switching once to email fallback. Confirm sender registration, consent, retention, suppression, and one-click unsubscribe obligations with counsel and current provider documentation; transactional authentication and marketplace marketing do not have identical rules. Estimate the engineering ownership of email code generation, expiry, replay defense, abuse controls, event polling, and reporting. Choose the SMS-first direct OTP design when those tests pass and template ownership is intentional. Stick with Twilio Verify or Vonage Verify when managed verification and additional channels matter more than a compact backend surface. Prefer AWS End User Messaging SMS when application-owned OTP logic and an AWS-centered operating model are already deliberate choices. If SMTP relay or a fully managed email OTP fallback is a hard requirement, this particular direct-SMS option is the wrong fit. No mystery there. References Twilio SMS documentation RFC 8058: Signaling One-Click Functionality for List Email Headers