Call pickup on BLF not working on 7.72.0.1 [resolved 7.72.0.56.rom]

[temp0001]

I have a number of T22P and T28P handsets configured with an Epygi Quadro 2x PBX.

A few years ago when setting up BLF on a DSS key of a phone (A T22P) to monitor a Reception extension (a T28P), I discovered it can also perform Call Pickup, which is an incredibly useful feature. It allows another office user extension to answer an incoming call at Reception when the receptionist is absent or away from her desk.

To my dismay, this feature stopped working after I upgraded the phone (that did the BLF monitoring) to 7.72.0.1. When the reception phone is ringing, the BLF button flashed as it did before, but pressing it results in a error message "Not found" or "Incomplete address". I spent hours messing around with the phone settings to no avail. The button still works when the remote extension is idle; in this case, it worked exactly as it did before as a speed dial.

I had to downgrade the phone back to 7.61.0.80 before Call Pickup on the BLF button would work again.

Interestingly, the destination phone (being monitored) can be on ver 7.7, but the phone performing the pickup needs to be 7.6.

Is there something special that I need to do on the 7.7 firmware to make Call Pickup on BLF work?