On a T22 with the 7.61.23.11 UK firmware, I have running with V11 official release -
I have the phone set with 2 different accounts at the moment. The second account is the one of interest (my account at the business). I placed a call to the system and let the IVR forward to my account whereupon I answered. I left the phone off-hook and dialed again using a cell and let the IVR forward to the extension. The rule is set to - if busy go to voice mail, which it did and I left a short test message which lit the message waiting indicator the second I hung up my cell (the second call). While still on the first call, I then placed it into park/orbit 1 and hung up the receiver. The MOH played while being in park. I then "lifted" the reciever and dialed the unpark code while "off-hook" and immediately was reconnected with my first call.
At least in my case, it seems that the functionality you seek is there and works fine. I have the line 1 key (account 1) set to one of my clients, the line 2 key to my extension at work, and the line 3 key set to monitor a 3rd, different client. I do not have a t26 at the moment, but I do have a t28 that I can pull out.
I also tested using the T22 with a VP2009. The Vp2009 performed as the T22 did above. I called in from the T22 using one of the clients accounts, and then left off hook, Called from cell and left VM, and got indicator on both as soon as cell hung up. Then placed t22 in Park from VP, hung up waited a little bit and then took VP receiver off hook and retrieved T22 with no effort. Only had to be certain that I was in the account of interest at the VP2009 has 3 other accounts besides the one at my business registered.
Am I testing the scenario correctly?