I have a Grandstream GXP280 sip telephone, which supports 1 SIP account. It is said that it supports two lines though, so one should be able to switch between calls on the same SIP account.
I tried this by calling to our DID, and picking up with the phone. Then I tried dialing the extension with an other phone inside the network. This gives me the busy tone though.
In my sip.conf I added busylevel and call-limit to the template of the receiving phone. This didn’t seem to help though.
[stumpel-zwaag](!) ; create template for our devices type=friend ; the channel driver will mathc on username first, IP second context=StumpelZwaag ; this is where calls from the device will enter the dialplan host=dynamic ; the device will register with asterisk ;nat=yes ; assume the device is behind nat secret=x ; a secure password for this device dtmfmode=auto ; accept touch-tones from devices, negotiated automatically disallow=all ; reset with voice codecs to accept from, and request to, the device allow=alaw ; which audio codecs we accept from allow=g722 busylevel=2 ; twee inkomende gesprekken call-limit=2