With the switchport you'll have to do some more troubleshooting. They can go bad - it's rare, but it happens. However before hitting that conclusion, does the data connection work when plugged in directly to the PC? If you move the phone to another jack does the switchport work there? If the phone works at other locations you have some networking (or the cable out of the passthrough port) to troubleshoot.
When you say "dial a number from voicemail" how is that happening?
The version of your software and platform you're on would be helpful to know too.