dial in over RRAS with Mobile failed

hi !!
we are a development Company for mobile Solutions. Just now we are testing connecting a WAP-Server on Windows2000 via RRAS and ISDN-Card. The Problem seems to be, that altough a session is established (authentication and accounting is ok ) nodata is arriving the WAP-Server. This problem we have only with Nokia Communicator, and Mobiles with phone.com Browser ( like Siemens or Motorolla ).No problem we have with all other Nokia and Ericsson Mobiles. If someone has experience with a situation like ours, please write a note. Everything would be very helpful for us.