Asterisk Installation:

http://wiki.vicidial.org/doku.php?id=consultants#india

Numbers:

Zadarma:
https://zadarma.com/en/support/instructions/asterisk/ 
USA $0.008 or unlimited 4 channels €32/month, SIP, API

VoIPberry:
https://voipberry.com/voip-rates/
USA $0.008 / minute. SIP

Quest Blue:
http://www.asterisksiptrunking.us/rates.html
USA $0.009