With Version 14 and under i was able to create a dummy extensions, lets call them 401/402/403/404. Then i would set the mobile number to be "SP1"/"SP2"/"SP3"/"SP4" and configure it so that when the extension was not registered it would forward to the mobile number. I would be then able to Dial 401 to pull a call out of SP1 or blind transfer a call to 401 to put the call into SP1. This was great for Yealink W52/56 phones and other phones that do not have the buttons to be able to configure a BLF key for Shared Park lines. this functionality was removed with V15 and i get an error saying that SP1 is not a supported Mobile Number format. Please bring this functionality back as it is very useful.