Nope that doesn’t do it. I just set up a test environment with freepbx 2.9, limited outbound concurrency to 1 on the extension but it still allows me to transfer the call I made. Thanks for the input though and in the process I discovered a new feature on 2.9!!
Without knowing exactly how the phone initiates a transfer it’s impossible to say if what you want to do can be done. And even if you knew, it might be a question about Asterisk rather than FreePBX, which probably isn’t getting involved in this at. I’m not saying that there isn’t anyone on this forum that would know the answer (I know I don’t) but it might help if you would mention the make and model of one of the phones involved (because someone might know how they do transfers), and also, if you can watch the CLI while a call is being transferred and see what Asterisk actually does at the moment of the transfer, it might give you (or a more knowledgeable person) a clue as to how it’s being done. Armed with that information, the Asterisk folks might be able to tell you if it’s even possible to do the type of blocking you want. If it’s not possible in Asterisk, then it’s not possible in FreePBX.