vonage mobile sip credentials
i have found the vonage mobile app on both android and iOS to have excellent audio quality and to be an overall great outbound calling service and completely free. the outbound caller ID is the number of whatever SMS receiving DID you use to verify the account, usually a mobile phone or GV account.
this post details how to obtain the SIP settings from a rooted android phone, you than use an alternative SIP client:
p.s. i have not checked but this may be against the terms of service from vonage.