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.
All you need to do is insure your CSS selector is more specific than the selector Skype uses. In the current skype style sheet they use:
span.skype_pnh_container span.skype_pnh_highlighting_inactive_common span.skype_pnh_text_span {...}
You will need to add an extra class — your own website context — to this selector, i.e.
.myclass span.skype_pnh_container span.skype_pnh_highlighting_inactive_common span.skype_pnh_text_span {...}