This is a good question since there are a lot of software available which can easily replace the Twilio service, so instead of monthly payments you can host it

This is a good question since there are a lot of software available which can easily replace the Twilio service, so instead of monthly payments you can host it yourself for much less. I think that Twilio is doing well only with small customers as the biggest customers could save a lot by self-hosting the same service. In fact you just need to install an Asterisk or some other Softswitch and on the client side use something like the mizu webphone which has all the Twilio's capabilities like API for call, video, chat, SMS, presence and others.