Music On Hold never stops after a transfer!

Here’s my problem,

I dial a SIP phone from a ZAP phone
and then the SIP user transfers the ZAP user to
another ZAP user. The transfer works correctly, but when the native bridge occurs

– Attempting native bridge of Zap/22-1 and Zap/16-1

the music on hold never stops. The two lines are bridged correctly and you
can hear one another but you essentially have to speak OVER the music on

When I do the same thing using three ZAP phones the music on hold stops correctly!

– Attempting native bridge of Zap/22-2 and Zap/16-1
– Stopped music on hold on Zap/20-1

Anyone knows what’s wrong?