14.5.6. Precedence Call Waiting 
Camping on by a lower or equal precedence call or by a call from a
different service domain
After dialing the access code and the extension number the text "PRECEDENCE
CALL" appears on the display of the originating user.
The destination user hears precedence call waiting tone.
When the destination user goes onhook, it will hear precedence call
ringing immediately.