Intermittent fax problem incoming only

100% of outgoing faxes work.  Client knows this because confirmations must be given for the contents of the fax.

Recently incoming faxes are intermittent.  The fax will ring every time, but not answer all calls.  The client bought a new brother fax and the same thing happens.  The old was a Brother also.  I haven't been onsite personally, but the tech that was sent out I talked with personally while he was there.  We believe the fax is answering the call and going silent at point of answering.  On site he could not find a setting to leave the connection handshake audible (may not exist).  The fax machine will record NG on the failed incoming.  Theses faxes are not large, so memory should not be the issue.  Line quality was tested on local calls and works perfectly.  The phone company has tested everything and agrees it works perfectly.  All local faxes work 100% incoming.  

So the client has reached out to the senders which have problems and none are using any digital fax (Spectrum service for example) or fax services.  They all use physical fax machines.  They are long distance calls and we suspect the connection to the local (Centurylink) exchange may be the problem.  Centurylink is still investigating this problem.  We just updated them this morning with additional fax # to check from.  The phone cord has been replaced to the fax.  Centurylink has a outside filter on the phone line which has a DSL piggyback.  They tested the filter and feel that is not an issue.  One other business in town is having the exact same issue with no luck yet.

What else could be the issue?