Here are my pjsip trunk settings for Callcentric. Everything not mentioned should be left at default. Replace 1777xxxxxxx with your Callcentric 1777 number.
Trunk Name: (as desired)
Outbound CallerID: 1777xxxxxxx (or a DID you have with Callcentric)
Username: 1777xxxxxxx
Secret: (your SIP password set at Callcentric)
SIP Server: sip.callcentric.net
Contact User: 1777xxxxxxx
From Domain: sip.callcentric.net
From User: 1777xxxxxxx
Match (Permit): 199.87.144.0/21,204.11.192.0/22
Send RPID/PAI: Both
Outbound Route should send 11-digit domestic numbers to CC. If your system allows other dialing formats, they should be converted to 11-digit format. 911 should be sent unmodified. You may need other entries for international, voicemail, special features, etc.
Inbound Route can use the 1777 number as DID Number, or see