I have a SoundPoint IP 430 running SIP Buffalo 3.2.7.0198. I used the phone to config "forward on busy". This resulted in the file "0004f2133ee5-phone.cfg" being created on the provisioning server with this:
<OVERRIDES reg.1.fwd.busy.contact="520" reg.1.fwd.busy.status="1"/>
The problem is that it doesn't work. If I'm having a conversation on this phone, and I call it from a 3rd extension, it still shows a 2nd call coming in, and the calling party hears ringing. The extension 520 does not ring.
By the way, forward on no answer does work. I haven't tried forward always, because that's not what I want. (This phone is in the conference room.)
it will work if the server is set to busy the extension out if a call is made on it.
Typically in SIP you may have to set the calls per line on the phone down to 1 otherwise in order to only allow one call.