Can't forward calls to AT&T CallVantage number

Archived from groups: comp.dcom.voice-over-ip (More info?)

Did anyone know or realize that you can't forward a cell phone for
example to your AT&T CallVantage number? If you do, the call will go
directly to voicemail and never ring the phone. I tested this about a
month or so ago and it would ring the phone about 1 out of 5 times and
the other 4 times it went to voicemail. I called AT&T tech support
today and they are now telling me that since it is an "on net voip
system", you can't forward phones to your CallVantage number and expect
it to ring! This is bogus (I think), can anyone explain technically why
it won't work? Otherwise I will be forced to believe that it is a
business decision AT&T made, so as not to tie up network resources...

Does AT&T competitors have the same issue?

Thank