A useful test for evaluating outbound voice quality is at 1-703-376-3246.
If the trouble is just low volume, there may a compatibility issue with the phone at your parents' home. Bring a phone of yours along and do a comparison test. I don't believe that a networking issue can cause low volume.
If you have noise or echo heard by the remote party, it's possible that the phone or wiring is causing some electrical noise that the OBi is seeing as constant outbound voice. If you are feeding their house wiring, test with one phone connected directly to the OBi, using a short cord.
If you have choppy voice, that is likely a network issue. Test by shutting down the OBi and making a call with Gmail (or a softphone, if the provider is other than Google Voice). If that's also bad, there is likely a problem with your line or modem, or you have interference from unintended traffic (malware, filesharing, neighbors leeching off unsecured Wi-Fi, etc.) Viewing the modem stats may show what's wrong.
If outbound voice with Gmail is ok, but it's still bad with the OBi, try running a continuous ping to the OBi from a PC on their LAN. If you see packet loss, try a different cable or router port.