I recently encountered a problem with my Verizon iPhone where any call that went to voicemail would instead get a system message saying the call could not be completed as dialed. Dialing *86 from the phone gave the same results. After much Googling, here's how I fixed it without calling tech support.
1. Dial *73, wait till you hear a couple short tones. Hang up.
2. Dial *900, wait till you hear a couple short tones. Hang up.
3. Dial *920, you know the drill. Hang up.
Step 1 clears any forwarding that may be on the phone.
Step 2 clears any forwarding for non answered call.
Step 3 clears any forwarding for busy calls.
This worked for me, but your mileage may vary.