Problems with "CUT"

Hello,

I have a problem with the cut function.
I want to extract a number from a P-Asserted Identity.
To example:
P-Asserted-Identity: sip:1234@1.1.1.1

I allready have the part:
sip:1234@1.1.1.1 saved in a var.

But how can I extract the phone number 1234 ?

Thank you very much…