Go ahead and default the switch. Put 78 and 79 in Hunt Group 7. Put an "auto-intercom" button for 78 and 79 on the phone you're using, so you can see if they actually answer. I have seen some that will answer, but not provide any audio. If they answer, you will see the light next to the auto intercom button come on.
If they answer, but you don't hear anything, try dialing *8-(an extension number)-#, and see if they can "hear" you. (This will transfer your call to another extension.)
If they can hear you, try blindly (or is it deafly) the default sequence for the voice mail card and see if that restores it. (Search on this forumn for the default code - I won't post it myself)