So this is odd. Couldn't see anything to help in the logs, but when unknown, blocked or anonymous calls and apaprently some 800 #s from what they tell me, come in, they cannot transfer the calls.
How far does the transfer get (referring to the 3CX logs)? Is this going back out via a VoIP provider? They may not like the name you are passing on to them.
I haven't been able to see it in action in the 3CX logs. Is there a way to look back further than what it shows through the web interface? It's going out through our SIP trunk with AT&T, not VoIP. AFAIK, it drops right when they try to transfer.
You are going to have to get a call to fail (call in blocking your number), collect the log and post it here. What type of set(s) is this happening with? The 3CX log may provide some clues. Do you have any other outgoing trunk(s) that you can test a transfer over?