Loss of outbound audio during a call (v1.2.3)

Ive run into a problem where during a call (VOIP), all of a sudden the person I called can?t hear me, but I can hear them no problem. It seems to do it randomly. Any ideas? Where should I start looking ? I sure could use the help . . .

Trixbox v1.2.3 (Asterisk 1.2.12.1)