We do lots of work with Twilio - it powers our political phone banking software and is the back-end for our virtual branch exchange - and of course we provide development/engineering services for those looking to integrate Twilio.
One significant issue we encounter with almost every new Twilio user is their mis-conceptions about how Twilio bills and what their costs will be.
The Penny per Minute Myth
When we had our main office line hosted on a true VoIP service the cost was $0.01/minute - yes - one cent per minute. We migrated to Twilio - now inbound calls ( which are generally forwarded to another phone number) are billed at $0.03/minute. Where did the other two cents come from?
Traditional providers charge only for one leg of the call - that is the inbound or outbound leg. This leg of the call originates/terminates from your VoIP system (we like FreeSWITCH). On Twilio however there are frequently two legs of the call.
The top example shows traditional VoIP operations - where the call goes to your server, or some "server" you operate and the other side is on your existing infrastructure. With Twilio however that "server" is them.
So, when using Twilio to route (forward) inbound calls to another device you'll be paying for both legs of the call. This is clearly visible in the billing reports from VBX - where the two legs are shown together.