ADSL Linking problem

Linking problem
We have a Local Area Network connected on NT platform, which was connected to our remote office in London through ISDN line, with a different IP address given to all the users , and we could replicate with our remote server using Lotus notes. But now the scenario has changed , we have changed our line to ADSL which is again connected through Cisco 677 router (Hope it is just a Digital modem), and we have a frequent complaints for accessing our mails , if we all log together. For us mails from remote server is very important . I feel that ADSL IP address is not static , so hope any of our experts among our group will be able to help me.

Thank you,

With regards,

Sylesh.