Asterisk
The backbone of open source telephony since 1999: a PBX, a SIP gateway and a toolkit for building voice applications. Distributed under GPLv2, sponsored by Sangoma, it runs on your own server.
Strengths
- Twenty-five years in production behind it: the ugly edge cases have already been hit by someone, and the answer exists somewhere
- Free under GPLv2, with long term certified releases (18.9, 20.7, 22.8) for deployments meant to last
- Your calls stay with you: conversation confidentiality is settled by design
- Extensible all the way down: dial plan, AGI and ARI interfaces, you wire your business logic straight onto call events
Limitations
- This is not an API, it is infrastructure: a dial plan to write, codecs, NAT and firewall traversal to sort out. Count in days, not hours
- Security is on you and the penalty is financial: a badly closed server becomes someone else calling on your dime, billed per minute
- One more server to operate: OS updates, audio quality monitoring, configuration backups, and nobody else to do it
- Open source does not remove the telecom bill: you still depend on a SIP carrier for numbers and minutes
- An old project with sprawling documentation split across six release branches: finding the right page assumes you already know what you are after
Best for
- An internal switchboard or call centre with routing that turnkey offerings cannot express
- Sectors where call recording and confidentiality are regulated and cannot leave your infrastructure
- High and steady call volumes, where per-minute billing on a managed API becomes the top cost line