Proof Holdings vs Sinch Verification
Sinch's flash call is real engineering — a missed call instead of an SMS, cheaper at scale where Android and carrier support line up. The catch hides in 'where it lines up': Android-only, pricing that shifts by country, and a pass/fail you can't take with you.
Where Sinch's flash call fits, it's clever. Proof's Reverse OTP works on any device in any country, from €0.0079 a proof — and every verification ends in a signed proof you keep, not a flag you lose.
One API · One key
One key does what a stack of vendors does
Every provider on this page delivers codes to phones. Proof is a single API — the same reverse-OTP flow also covers email and domains, and every verification mints a signed, offline-verifiable ES256 token for each of these:
Same price · Any asset
The same price for every asset — phone, email, and domain today, with wallet and social coming soon. No per-seat or per-MAU fees — the meters that make rivals' bills jump.
- Free every month
- 300 proofs
- Pay as you go
- €0.03 / proof
- At volume
- €0.0079 / proof on the €79 plan
Feature Comparison
Swipe to compare →
| Feature | Proof Holdings | Sinch Verification |
|---|---|---|
| Phone verification | Telegram, WhatsApp, SMS | SMS, Flash Call, Phone Call, Data |
| Email verification | Magic Link, OTP | No |
| Domain verification | DNS TXT, HTTP, Admin Email, Auto-Verify | No |
| Wallet verification | Coming soon (8 chains planned) | No |
| Social verification | Coming soon (12+ platforms planned) | No |
| Reverse OTP | Yes | No |
| Flash call verification | No | Android only, caller-ID based |
| Cryptographic proof tokens | ES256 JWT, offline-verifiable | No |
| Human-in-the-loop approvals | Telegram/WhatsApp with proof tokens | No |
| MCP server for AI agents | 167 tools | MCP server available |
| Code delivery | No code to deliver (user sends it) | SMS codes can fail to arrive; flash call helps where it works |
| No code in transit to intercept or phish | Yes | No |
| Free tier | 300 proofs/month | No |
Pricing
Cost per verification
Proof Holdings
From €0.0079/proof (any asset, any country)
Sinch Verification
Country-variable, not publicly listed (requires Sinch account to view rates)
International pricing
Proof Holdings
The same price in every country
Sinch Verification
Varies by country: $0.05-0.50+ per SMS internationally
Free tier
Proof Holdings
300 proofs/month
Sinch Verification
No free tier
Multi-asset pricing
Proof Holdings
Same price for phone, email, domain (wallet & social soon)
Sinch Verification
Phone only — no email, domain, wallet, or social
The Hidden Cost: Country-Variable Pricing
Sinch advertises $0.01-0.06 per verification request. But this is the platform fee only. On top of that, you pay SMS channel fees that vary dramatically by destination country.
An SMS to the US costs ~$0.01-0.06 depending on provider. To France, $0.08 (Plivo). To Netherlands, €0.08 (Infobip). To some countries, $0.20-0.50+. The advertised price and the real price are very different things.
Proof Holdings charges from €0.0079 per proof regardless of country or asset type. Phone verification to any country, plus email and domain — all at the same price (wallet & social coming soon). No country surcharges, no channel fees, no surprises.
Flash Call vs Reverse OTP
Sinch's flash call is a missed-call verification: the service calls the user's phone, the call auto-hangs-up, and the verification code is embedded in the caller ID. The user's phone reads it automatically.
Flash call is clever but limited: Android only (iOS blocks caller-ID reading), requires phone permission, and depends on carrier support. It doesn't work in all countries.
Proof Holdings' Reverse OTP works on any device, any OS, any country where the user has Telegram, WhatsApp, or SMS. The user sends a code to the service — no code to deliver, no OS restrictions.
Security & Proof Tokens
Both Sinch's SMS and flash call channels are vulnerable to SIM-swap attacks. If an attacker controls the phone number, they receive the SMS or flash call.
Reverse OTP sends no code to intercept or phish, and on Telegram and WhatsApp a SIM swap alone is not enough — the attacker must also take over the messaging account, which two-step verification blocks when the user has turned it on.
Every Proof Holdings verification produces an ES256-signed JWT proof token verifiable offline. Sinch returns a verification status with no cryptographic attestation.
When to use Sinch Verification
- You want flash call verification for Android users (lower cost than SMS in some markets)
- Your use case is strictly phone-only verification
- You need Sinch's broader CPaaS platform (SMS, voice, video)
- Flash call carrier coverage is good in your target markets
When to use Proof Holdings
- You need to verify more than phone numbers (email, domain today; wallet & social soon)
- You want predictable pricing with no country surcharges
- You need verification that works on any device and OS
- You want cryptographic proof tokens for every verification
- You need human-in-the-loop approvals for AI agents
- You operate internationally and want no codes lost in delivery
Frequently Asked Questions
Ready to try Proof Holdings?
300 free proofs per month. No credit card required.
Building with an AI agent? The 167-tool MCP server and test mode let an agent run a real verification right now — machine quickstart in llms-full.txt.