Originally Posted by iParanormal
Grandstreams still use pots lines. If you're having things defaulted and still getting the issue, could be an issue with the wiring from the grandstream out rather than on your network. Or even a provider-level issue.
Kind of what I was thinking as well, since there's multiple businesses in the same building.
Make sure the VOIP is configured correctly, and then if it still does it contact the ISP.