When using Windows Messenger communication through the Internet, the IP address of the computer must be a global IP address. The terminal gets a global IP address from the ISP provider when using dial-up connection via a modem or a terminal adaptor (TA). Therefore, there will be no problem with the features of Windows Messenger.
On the other hand, when Internet connection is through an ISDN router or an ADSL modem (router type), a global IP address is set to the ISDN router or the ADSL modem by the provider, and a private IP address is assigned to the computer by the ISDN router or the ADSL modem. In this case, the features of Windows Messenger will be limited.
However, some ISDN routers or ADSL modems can overcome this symptom. Please approach the respective manufacturers for more details.