P-asserted id

[maria.para]

Hello,

In an oxo R920 I have add a public sip trunk . The provider doesn 't need authentication.
The incoming calls work fine but i have a little problem with the outgoing calls. I can make external national calls but no calls to mobile numbers , to short numbers,etc.
The provider told me to add "+" in P asserted id header.

Is this something that i can do ? If yes do you know what excactly i have to programm in oxo?

Thank you ,

Maria