Blind Transfer sometimes not working

I’m having an issue where the blind transfer feature of FreePBX is sometimes not working. In each case, I am calling from the same number, to the same extension and transferring to the same external number.

The external number will receive the call and when answered there is no audio.
The calling number will hear MOH once the transfer is started and continues hearing MOH until the call is hung up.

Examples attachedexamples.tgz (17.0 KB)

I’ve narrowed down this issue to how the desk phone (extension on the PBX) is operating.

If I initiate a transfer, dial the number, and send the transfer in rapid secession, then the phone will transfer the call with a REFER method, providing the new SIP URI as the new destination - the PBX handles this just fine.

If I initiate a transfer, dial the number, and wait, the phone will initiate a new call out and allow me to perform an attended transfer. Once I’ve “talked to the person”, if I then press the transfer button to complete the transfer, the phone will send out a REFER method and provide the Call-ID, To tag and From tag. The PBX handles this just fine.

The problem is when I wait just a little bit too long when intending to do a blind transfer. The phone thinks I’m trying to do an attended transfer and begins initiating the call out. If I press the transfer button before the call has fully set up, then the phone is unable to connect those two calls together.

I’m guessing this happens because the phone does not have yet have the To or From tag.

The fix for me has been to extend the wait timer when a dial pattern has been matched. It was set to 1 second (send the call “now” because we’ve matched a dial pattern); I have set the timer to 4 seconds - giving the human element more time to press the transfer button when initiating a blind transfer.

On what type of phone are you seeing this?

Sangoma S500, Firmware: and

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