Dial-in settings keep changing

Whenever any of the users on this PC want to dial in, the dial-in
settings have been changed and need to be manually reset to the original
values; the last three characters of the original username are
misteriously replaced by: 7.165.58039 and the telephone number to
002463615082. The password stays as it was. We have tried to change
these settings in all sorts of places, but they keep getting set back to
these values. I've run AdAware a couple of times and keep Sophos
Antivirus updated to check for viruses but nothing seems to be able to
fix the problem. Could someone help please?

Regards,

--
Rouke