Partner ACS R7 w/Verizon Fiber Box - Lines are Locking Up Repeatedly

I have a site with a Partner ACS R7 w/two 308ECs and a PC Large Voicemail card. The site was recently told by their line provider that their copper lines were being "sunsetted", and the site was converted to fiber (I believe via a Verizon ONT).

Since the site was converted, their Partner ACS has continually been locking up Lines 1 & 2, and the only resolution that's worked so far is to reboot the system. In addition, the fax line that's not running through the phone system also locks up and prevents faxes. Of course, the provider is insisting it must be the phone system that's at fault, despite working normally right up until the POTS lines were converted. We've checked the processor and the configuration is the same, no programming has been lost, etc.

Does anyone have any similar experiences with solutions they can share? One of the techs that I spoke to onsite mentioned that he had seen a similar situation where the ONT device needed some tweaking, and I'm wondering if that's a possibility. The site is worried they'll have to spend money to get a new phone system if they can't come up with a solution. Thanks in advance!