I'm not familiar with CallWithUs, but you can do it with CallCentric.
Get a free IP Freedom account here:
https://www.callcentric.com/products/OBi110
Assuming your VOIP provider is defined on SP1
Line InboundCallRoute:
sp1(1777xxxxxxx@in.callcentric.com),ph
1777xxxxxxx is your Callcentric account#
Service Providers -> ITSP Profile A SIP -> X_SpoofCallerID: Checked
OBi200
Define your Callcentric account on SP4.
You can use the OBiTalk wizard to define Callcentric, it's one of the options.
Voice Services -> SP4 services -> X_InboundCallRoute:
{(callerid1|callerid2|callerid3):aa},{ph}
Replace callerid1-3 with your trusted caller's CallerIDs.
Trusted callers will be routed to the Auto Attendant.