I found this while I was online poking around. TrapCall - Unmask Blocked Calls It works. SOURCE: Annoyances: TrapCall Reveals Who's Behind Blocked Calls
I don't see how that can happen if you set the CF on both rejected and unanswered calls. If you only set it for rejected, then yes, it can reach the VM. If you set it for both might as well use one of their paid packages that includes VM.
I tried the service, however, the service doesn't work on T-Mobile prepaid (at least for the ToGo accounts) since call forwarding is disabled on these types of accounts.:O I called TMo prepaid CS and I was given the sad news. If you have FlexPay or monthly service, it should work since call forwarding is enabled to those accounts.