I get "Recall" when doing attended transfer from EXP20 + T29G

[shifa]

I have an issue after connecting an EXP20 Expansion Module to a Yealink T29G. I have setup the EXP20 with:

- BLF extensions on one screen
- 8 LINES on the second screen (4 lines for each DID, there are two DID's with ring groups attached to this handset)

The problem occurs when receiving a call on this handset, then placing it on hold, then attended transfer to another (BLF) extension - the other BLF extension user picks up the phone and we can talk, but when I close the handset or press transfer to complete the attended transfer, my phone appears with "recall" and the transfer fails (the original caller is back online)

I have tested with the EXP20 module to ensure all the BLF's are defaulted to use Line 16 (which is a totally separate extension that does not ring on the DID ring groups), but I still face the same problem.

I have spoken to my VOIP company who offered the above solution but it didn't seem to work.

What do you suggest?