Not sure I would say that. They have voice, video, messaging, SIP, etc. They have a lot of tricks.
Twilio is the market leader for developer SMS, and voice.
They are not the market leader for any of the other categories that they are trying to expand to.
Edit: if I recall correctly from many years ago, this is who Twilio uses for their US telecom/CLEC interface.
Edit 2: yep, bandwidth.com now even competes against Twilio: https://getvoip.com/amp/blog/2016/06/01/twilio-alternatives/
Twilio is good at one thing. And that is the developer experience.
If you are willing to pay 100X the price, in order to have awesome documentation, then Twilio is the right solution for you.
Otherwise.... Well.... Anything else is probably better.
[0] https://www.twilio.com/sip-trunking/pricing [1] https://www.flowroute.com/pricing/