Help Google voice sometimes sends missed calls to VZ error message

[Clocks]

I used google voice for voicemail, but still use my old verizon phone number.

Sometimes when a call goes to voicemail the caller will hear "Welcome to verizon wireless. The number you have dialed has been changed, disconnected, or no longer in service...."

Any ideas? My phone says "Voicemail Service: Google Voice" in Call Settings, and I can call and check my google voice voicemail just fine.