Following a recent firmware to Virgin Media's "Super Hub" to version R36 I've discovered I can no longer use the "transfer" key on our desk phones to initiate a attended/supervised transfer (where you introduce the caller and then transfer the call).
A bit of back story…
We've got several Linksys SPA941 and SPA942 VOIP phones using SIP and, while I can make and receive calls, when I try using the unattended transfer button on the phone (that uses SIP "REFER" messages to initiate the transfer) the transfer just fails. Unattended (or "blind") transfers work fine.
I've tried setting the phone's IP in the DMZ, port forwarding, enabling/disabling the firewall, etc and nothing makes it work – nothing on the VOIP server (an Asterisk server which we control) has changed since February, and the phones were working fine until the R36 update a few weeks ago.
We ended up working around the problem by using feature codes to call Asterisk's built-in transfer functions (which completes the actual transfer on the server – which isn't behind the Super Hub – rather than the phone) but it's still a bit of a pain having to dial a feature code rather than just being able to press the transfer button on the phones.