High-pitched squeal when transferring calls to outside line

We have a FreePBX system running with a Digium TDM 800 card. This card has 5 analog ports active with hardware echo cancellation. Everything seems to run great, except when calls are transferred to an outside number. If the receptionist receives a phone call, answers, and attempts to transfer to call to an outside number (transfer, 218-555-5555) the phone will ring, and the outside party will answer. All while the person who called in is listening to music on hold. However, once the receptionist hangs up the phone to finish transferring the call, the party who called in will now receive a high pitched scream, as if microphone feedback is coming in through the phone line. This lasts indefinitely and also happens with blind transfers to outside lines. However, if I create ring groups that dial external numbers using 2185555555# in the ring group, the calls go through perfectly. Internal transfers work just fine as well. The phones being used are Polycom 501’s.

Has anyone experienced this before? How do I fix this?