Twilio
The reference telephony API: SMS, voice calls, WhatsApp and code verification, without ever touching a carrier or a SIP server.
Strengths
- Documentation and SDKs among the best available, first message sent within minutes
- Worldwide carrier coverage and compliance handled for you, which is the real work behind an SMS
- A single provider for SMS, voice, WhatsApp and code verification, instead of four integrations
- No infrastructure to operate: no SIP server, no carrier contract, no on-call rotation
Limitations
- The listed price is not the price paid: carrier fees stack on top, message by message. In the US, AT&T for instance adds $0.0035 on an outbound long-code SMS. At volume, the gap between quote and invoice is structural.
- In the US, A2P 10DLC registration imposes fees and a validation delay before you can send a single application message. That is not a billing detail, it is a go-live blocker.
- Usage billing with no hard cap: a bug looping on a send does not trip a guardrail, it triggers an invoice.
- Leaving Twilio means rewriting the whole telephony layer and renumbering, because numbers leased from them are not all portable without friction. The exit cost is real, and it grows with the time spent there.
Best for
- Early-stage products sending low volumes with no team to operate telecom infrastructure
- Worldwide use cases where you would otherwise negotiate with a different carrier per country
- Code verification and two-factor auth, where delivery reliability matters more than unit cost