If you are unable to dial into a voicemail box perform the following actions:
Make sure that the TalkSwitch is on the most recent version of firmware. If it is not, you will need to update it. If the firmware is updated than you will need to reboot the TalkSwitch system. Do this within the TalkSwitch Configuration Software by clicking Tools --> Reboot TalkSwitch...
After the TalkSwitch has finished rebooting try dialing into the voicemail box to make sure it is working.