A couple of quick thoughts, Michael.
If you are running a database kind of Program it may be treating the data you are entering as numbers and not text. Can you reformat that particular field to recognise all data as text, even if you are inputting numbers?
And also, some Programs expect telephone numbers to be inserted in a particular format - usually US. In Windows 2000 dial-up networking you have to un-check "use Dialing Rules". Check that out.