A minimal configuration of CallWithUs would be (from factory defaults):
Service Providers -> ITSP Profile A -> General -> STUNEnable : (checked)
Service Providers -> ITSP Profile A -> General -> STUNServer :
stun.callwithus.comService Providers -> ITSP Profile A -> SIP -> ProxyServer :
sip.callwithus.comService Providers -> ITSP Profile A -> SIP -> RegistrationPeriod : 120
Voice Services -> SP1 Service -> AuthUserName : username
Voice Services -> SP1 Service -> AuthPassword : password
Physical Interfaces -> PHONE Port -> PrimaryLine : SP1 Service