Most phone browsers will allow your "<style>" tags to be applied when the user views your email. This gives good control over the display of the signature.
Most phones will recognise a phone number, and format it as a link. Adding the styles above should sort these out in most cases, but if some email clients are still formatting them as links you might have to style the phone number itself.
To override this formatting, turn the phone number into a link, and format it directly as shown below. If you still want the user to be able to click the link, and make a phone call from it, do not include an href attribute. If you want to disable the click-to-call ability, set href="" which will make it text only.