Are you using an alternate solution (like Google Voice or Visual Voice mail, etc)?
Other than some 3rd party application causing a conflict I can't think of a reason this might be happening.
Have you tried the usual troubleshooting steps?
1. Restart phone
2. Shut down/battery pull
3. Last resort - factory reset
Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM -0.001 using Tapatalk 2