Coding a form field

Hi all - I am a Wix newbie with about 25 sites under my belt so far… always looking to learn and I think coding is very interesting, I’ve not needed to use it before now though!

I have a Wix Form on a site and unfortunately the standard phone number field is not numerical only, and I need it to be. Customer doesn’t want to pay someone to go in and code one thing so I am looking in to how to do it - dipping my toes in so to speak!

Can anyone help me with how I would go about doing this? I’m sure it’s super simple to everyone here but I’ve no idea where to start, so please be gentle!
Many thanks in advance