Are they calling your local number from a land-line, or mobile in the Philippines, coming in through your VoIP provider trunk, or, are they using an extension off your PBX (3CX App)?
If it is from a "normal" phone, that happens to be located in the Philippines, and you have had no other audio issues over your VoIP trunks, from other locations, then it is probably a transport issue beyond your provider, most likely in the Philippines. They only way to pinpoint the issue would be to replicate, with your VoIP provider involved, and to attempt to trace the call, which is unlikely to happen.
The caller would probably be better off reporting the problem at their end.
If this were a trend, and you had a number of calls, with no audio, from various locations, then I would consider, that it might be a 3CX issue.