mintfax vs. Sinch (Phaxio v3)

The headline gap

Phaxio v2.1 (2011-2022) was the developer-favorite fax API of its era. HMAC-SHA1 signed callbacks via X-Phaxio-Signature. Magic test number +19898989898. A self-serve sandbox. Clean docs. Then Phaxio was acquired by Voyant and absorbed into Sinch, and v3 happened.

Sinch’s v2-to-v3 migration guide says it directly: “Webhook signatures: Removed due to low usage, at the moment V3 does not support webhook signatures, if you are interesting in this feature or any other security method for us calling your server reach out to me [email protected].” Migration also broke multi-recipient sends (each recipient now becomes a separate fax ID), the API renamed every field from snake_case to camelCase, and every Phaxio v2.1 customer had to create a new Sinch account - it wasn’t an upgrade path.

mintfax inherits the slot Phaxio v2.1 vacated. HMAC-SHA256 webhook signatures (stronger than the SHA-1 Phaxio used). Idempotency keys. Built-in retries. Magic test numbers. Self-serve sandbox. Free BAA.

Feature comparison

Verified against Sinch product and developer documentation, May 2026.

What Sinch (Phaxio) still does well

mintfax doesn’t ship HTML-to-fax rendering. Phaxio’s WebKit-rendered HTML-to-fax is a real, useful feature - if your application is templating cover pages or rendering dynamic content, Phaxio still does that better than mintfax does today. It’s on our roadmap, but it’s not in production.

Sinch has 200,000+ business customers across its broader CPaaS platform. The Phaxio fax API has been in production for 14 years. Brand continuity, scale of operation, and depth of carrier relationships are real assets. If your team values “they’ve been doing this forever” over “they ship the security primitives we need,” Sinch may be the right call.

PhaxCode (barcode auto-routing on cover pages) is a clever feature that mintfax doesn’t ship and probably won’t.

The migration story

mintfax and Sinch both speak REST/JSON. The fax submission shape is similar enough that the code rewrite is small: change the base URL, the auth header format, and the field-naming convention. The bigger lift is reintroducing the security primitives Sinch dropped.

What changes for the dev:

• Base URL: https://fax.api.sinch.com/v3/projects/{project_id}/... becomes https://api.mintfax.com/v1/...
• Authentication: Sinch uses HTTP Basic with a project-scoped key + secret. mintfax uses bearer tokens.
• Webhook handling: This is the meaningful work. If your current Sinch v3 integration relied on HTTP Basic in the webhook URL for “security,” replace it with mintfax’s HMAC-SHA256 verification. Most teams report this taking under an hour.
• Field naming: mintfax uses snake_case consistent with v2.1-era convention.
• Multi-recipient sends: mintfax restores the v2.1 shape: one submission, one fax ID, multiple recipients (subject to per-fax volume limits).