Call transfer and MOH

Hello everyone, I have a problem with MOH and call transfer, I’ll explain better:

  1. secretary sip B calls secretary sip C;
  2. secretary sip B transfers with waiting and calls director sip A;
  3. secretary sip C transfers with waiting and calls director sip D;
  4. secretary sip B hangs up by transferring the director sip A on the channel of the secretary sip C.

At this point, in a totally random manner, the director sip A sometimes does not hear the MOH when the secretary Sip C is still talking to the director sip D.

The tests that gave these alternate results were performed with:

asterisk 1.8
asterisk 11
asterisk 13
asterisk 15

on both virtual and physical servers with OS centos ver. 6X and 7X and fedora ver.27

with terminals:

snom 710 and 720
polycom vvx 600
x-lite softphone, microsip, zoiper

the sip and pjsip protocols were used in the tests.

Has this problem ever happened to you? is there a “bug” or a case documented for this problem?

thanks to all