Cannot hear or see call waiting

[Supportvzla]

I have a panasonic Ease Phone KXT61610B
When I receive a external call, I can see and hear the call waiting indication when a second external call arrives.
But when one of the call (or both) are internal I dont get nothing, and the person that is calling me gets busy: For example If extension 12 calls 11, when ext 13 calls 11 gets a busy signal
All phones have: "TONE 1" (call waiting tone), "TONE CALL" Intercom Alerting Mode), "EX-ON CO-ON" (Program. Call Waiting)

supportvzla

 

[phones001]

On the KX-T61610 the call waiting from internal party in NOT automatic. The calling party must dial the digit 1 when they hear the busy. They then start to hear ring back and can wait for you to answer.