Random HP faxing problems..

bit of a long shot this.. but im having problems with a bunch of HP M4345MFP printers in the company.. they are big multifunction printers/scanners/faxes etc

Anyway, we seem to be having an odd problem with them regarding faxing.

when sending/reciving faxes to/on certain numbers the fax will fail.. usually with "communication failure" - in some circumstances the fax will still be sent, but be shown as failed..

Ive updated the firmware.. but this doesnt help.. theres several fax settings but i have no idea what they do, also some printers keep printing T-30 Trace reports with every failed fax..

Anyone a fax guru? lol