One of 3 fields must be provided

What I would do is create a radio button field for "Phone Number" and have the options be:

• Home
• Mobile
• Work

Then, add three required telephone fields, Home, Mobile and Work, and make each choice conditional on the radio button selection. That way, they need to tell you what type of phone number they are providing, and the field for their phone input will be shown based on their choice. In the admin, you will have 4 fields then, one the choice of phone (home/mobile/work) and then one of the other 3 fields will hold the phone number.

You don't have to use 3 fields for the phone number if you don't want to. Just use the radio buttons for them to tell you which sort of phone number they are providing, then the phone number is in the next field. Make both of those fields required.

Will one of those approaches work for you?