As far as I know, the Attended Transfer does not work reliably on the Snom 3xx IP phones with PBXes. There are at least 2 ways the Snom phones can perform an attended transfer, based on the information in their WiKi, so there is some ambiguity as to how to properly perform an attended transfer on a Snom phone.
On a Snom 360 running the 7.3.30 firmware version I was able to transfer an inbound call from one PBXes extension to another PBXes extension via the following steps:
• Answer the inbound call on the Snom.
• Put the call on Hold, using the Hold button.
• Dial the extension I intended to transfer the call to.
• Talk to the person I called.
• Press the Transfer button once, and then a second time for the call to be transfered.
Try it and hopefully it will work for you as well.