Zap attended transfer problem

i am using freepbx 2.3.1 and asterisk 1.4.15.
i test the following situaton:

  1. zap/1 call sip/1
  2. sip/1 answer and press *2 for attended transfer, and call sip/2
  3. zap/1 now hearing music on hold, which is good.
  4. sip/1 hang-up before sip/2 pick up.(so now asterisk is on blind transfer mode)
  5. sip/2 keep ringinig, which is good.
  6. zap/1 leave music on hold, but didn’t hear ring.(which is bad)

i want to ring zap/1 at step 6. how can i fix this?
thanks a lot for your help!!

PS: if all theree phone are sip phone, then everything is ok. sip phone can hear ring when attended transfer become blind transfer

i miss the testing. i thought it’s ok when all phones are sip. but the same situation happen when all phones are sip.
i just don’t know if the problem is at asterisk, or at freepbx?
attended-> blind transfer cause the caller hear the air.

thanks for your help!!