I've never setup a dummy SIP trunk.
This is how ianobi does it:
QuoteRemember SP2 must be configured for SIP. If you don't have a SIP provider and SP2 is unused, the following will enable SP2 for SIP:
Service Providers -> ITSP Profile B -> SIP -> ProxyServer : 127.0.0.1
Voice Services -> SP2 Service -> AuthUserName : (any userid)
Voice Services -> SP2 Service -> X_RegisterEnable : (unchecked)
Voice Services -> SP2 Service -> X_ServProvProfile : B
Also, if you want CallerID passed do this:
Service Providers -> ITSP Profile B -> SIP -> Check (enable) X_SpoofCallerID
Some providers don't support CallerID spoofing. Sip2Sip does support it.