According to callwithus.com SIP configuration:

SIP server (or proxy, or domain) sip.callwithus.com
SIP proxy (or “Outbound Proxy”) leave blank
STUN server stun.callwithus.com
Username (or User ID) username
Password password
Auth name (or Auth ID) username
Display Name (used for callerId information when you place a call) Your name
Register (or Send registration request) Yes
Silence suppression (or Voice Activity Detection) On
Use DNS SRV Yes

How to setup Voicent SIP according to the above info?

Please use the following for setting up Voicent SIP:

STUN Server: stun.callwithus.co

SIP ID: sip:466000000@sip.callwithus.com
SIP Proxy: sip.callwithus.com
User: 466000000
Password: {your password}
Realm: sip.callwithus.com

Please replace 466000000 with your callwithus user name.