Returns 'failed' with 'cause' = 'invalid' if SIP address is malformed.
Returns 'failed' with 'cause' = 'invalid' if MAC address is malformed.
Returns 'failed' with 'cause' = 'nonexistant' on no such registration.
Returns 'failed' with 'cause' = 'invalid' if SIP address is malformed.
Returns 'failed' with 'cause' = 'invalid' if MAC address is malformed.
Returns 'failed' with 'cause' = 'nonexistant' on no such registration.