After upgrading to 2.5.0, a “text box” sometimes becomes a “telephone box”

After upgrading UM from 2.4.2 to 2.5.0, a “text box” becomes a “telephone box”, but not always :

In the members’ profile form I have created 2 phone numbers : a fixed phone (9 digits plus some spaces) and a mobile phone (10 digits plus some spaces). Both fields were defined as “text box” in UM 2.2.
Now, after upgrading UM to 2.5.0, the mobile phone becomes a “telephone box” : in the front end, you can click on it and a phone call can start. This is nice !
The fixed phone is still a “text box”.

Why is this different ?

I want to have the fixed phone also as a “telephone box”. How to do that ? I can not change the field type, and if I delete the field and create it again as “telephone box”, I get an error : “Your meta key already exists in your fields list”. Can I delete the meta key of the deleted field ?