FastCall support a SIP phone as a user phone in click-to-call. The SIP address is added as a user phone. Like the softphone, FastCall dials the user on the SIP phone directly. The SIP phone is $.01 per minute less with Twilio and tends to be faster than click-to-call.
NOTE: SIP phone is a great option but must be setup by your internal IT team. This is one FastCall feature that is not simple to setup.