Hi Istvan,
Cisco routers can work in both Enbloc and Overlap modes as explained in the following doc
http://www.cisco.com/c/en/us/support/docs/dial-access/dial-on-demand-routing-ddr/23382-isdn-overlap-prob.html
You can either check with your Telco if they can send the DNIS as per your requirement, else you always have the option to configure your gateway accordingly.
HTH
Manish