T28P incoming call bip when already on line

[julien]

Hello,
my users are complaining about something very interesting:

When they are on line with someone and there is another incoming call, they have one bip that lets them know about the incoming line but that bip doesn't get replayed as long the person is waiting on the other line.

I cannot find a way to make that signal last as long as no one picks up the line ...

Could you please let me know how to do that ?

Thank you very much
Julien