Get a free Callcentric Freedom account here:
http://www.callcentric.com/products/In your Callcentric account:
Click "Call Treatments"
Click "Add a new Call Treatment"
Select "Busy tone"
Click "Save"
Use {(NumberToBlock):spx(17771234567@in.callcentric.com;ui=$1)},{ph}
17771234567 is your Callcentric account number.
spx must point to a SP trunk (SP1 or SP2) defined as SIP.
If you don't have an SIP trunk define a dummy SP2 like this:
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