Asking for a phone number on your form: how to do it well

The quality of your list is decided at the moment somebody types their number. Four things remove most of the errors there.

Validate immediately

Check the format while they type and show a clear message on error. Correcting afterwards is impossible, because you cannot reach the person.

Ask for the right format

Show an example with the country code. Without an example you get five different spellings in the same column.

Say what you will use it for

One sentence next to the field. That raises the completion rate, because otherwise people fear being called by a salesperson.

Do not make it mandatory if you do not need it

A mandatory number field produces invented numbers. Better a smaller set of real numbers than a larger set of unusable ones.

Check on save

Format validation catches typos; a check against the network catches numbers that do not exist. For lists you will send to later, that second check is worth it.