On those Windows machines with Skype installed, it tends to convert all phone-formatted numbers to Skype links so you can click it in order to make a call on Skype.
The official solution is to add the following meta tag to the head section of your webpage. This will prevent skype from doing any reformatting on phone numbers.
<meta name="SKYPE_TOOLBAR" content="SKYPE_TOOLBAR_PARSER_COMPATIBLE" />
This is a vendor-specific tag. See Skype Toolbar meta tag
I just found that if you use the new HTML5 tel protocol it blocks the Skype junk from appearing:
<a href="tel:+18001234567">1 800 123 4567</a>