You would have to add it to your iOS/Android phonebook. There have been reports that Android can not do this.
Another way is create an extension (I use 771, 772, etc - 77 for SP - easy to remember.)
Set the email to my own. Send me an invite. Provision my softphone. Set the extension to forward calls to SP? when not registered.
Do this process for each SPx.
When done, reprovision softphone for your own extension.
Not the SP extensions are not registered.
On iOS, Android - dial the new extension and voila.
There are NO BLF's in the manner of a traditional SIP phone.