Asterisk match incoming calls

Hi Guys,

i have 1 SIP extention and 1 SIP Trunk( for isdn calls) from the same device.
If i get a call from isdn over the SIP Trunk, i get a username mismatch, have <299>, digest has <isdntrunk>.

Why is the asterisk thinking it´s the SIP extention?

regards
mothertrucker