Hi, I am Dave, I will help you with this.
All protocols are stored in the registry, do you want to reset that back to no app set for that protocol?
Click your Start Button, type regedit and hit Enter to open the Registry Editor
Click View and make sure 'Address Bar' is turned on
Paste this into the Address Bar at the top and hit Enter
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts
In the left pane that folder will contain all the different file extensions recognised by Windows scroll down to find .tel
When you find that in the left pane, right click that .tel key (folder) and choose Delete
Close the Registry Editor and restart (not shut down) your PC