You stated at the beginning this is an "intermittent problem". If this doesn't happen to every call transferred to the Single Line Extension I would guess there is something wrong with the way one of the EVX Ports is programmed in the A Screen. When the call gets transferred by this port, the problem shows up. I always program the A Screen for the EVX Ports as follows:
Line Ringing Day and Night all Lines No
Camp On Timer 000*10s
Hunt Group 36
Page Zone 000
Ring Type 2
Hold Recall 099*10s
Transfer Recall 099*10s
All other Timers 000
Auto Answer Y
Blk Barge Tone Y
VMS/Attend Y
All others N
Program 2 0300(100) and 0000(000) for all of the other keys.
Also check the G Screen. The EVX extensions should not be forwarded anywhere.
Hope this helps.