If you turned fax relay off on the vg248, make sure that the relay is also turned off accross the board, i.e. it should be turned off on your gateway as well. if it is an h323 gateway, just go to the dial peer for your voice and do it there, if you have an mgcp such as a 6608, then you should uncheck the fax parameters like ECM enable and fax relay enable. that shoudl take care of it. On ATA's, go to its config page by /dev and change the audio mode to 0x0000012 (not sure how many zeros though), try it and should work.
