Hello @FirstNameL43000 ,
At this time the link insert function doesn't allow for manual tel: additions. That said, most email mobile apps will automatically overwrite plain-text phone numbers to convert them into "links." Additionally, if you have a linked telephone number you're able to copy from elsewhere, it can be inserted into the email and has been known to work sporadically. Please bear in mind that some email clients have been known to treat emails with such links already inserted as suspicious.
I've attached the relevant feature request to your case and submitted it to the engineers. If and when they have an update regarding the development of this feature, they'll make sure to notify you.