tel is a valid URI scheme;
tel: works in the same way
Microsoft Office (including Word and Outlook) will automatically change
tel:something into a link. You can disable it following these instructions (I'll add them below, too). However, this link is inserted by the sender. Your client is merely displaying that link as the sender intended. You'll have to contact the sender and get them to stop, which can mean turning off automatic hyperlinks if using Outlook. Of course, the easy (and recommended) way is to just remove the hyperlinks before sending.
If it really bugs you and the sender can't or won't change it, you can disable HTML messages in Outlook. That's a rather drastic action, though. It might also be possible to set up a spam filter to strip links from this sender. That will likely remove all hyperlinks, not just
Instructions to disable automatic hyperlinking
Note: the easy way is to just remove the hyperlinks before sending.
(Screenshots from Word 2010, but should work the same way in Outlook)
Basically, go to autocorrect options, autoformat as you type, and uncheck the hyperlinks option. Warning: this will also turn off automatic