Limiting / Validating A Phone Number Field

Hi.

I'm trying to create a Phone Number field in a Pre-Chat Survey and limit it's entry.

I need to limit this field to 10 numerical-only characters.

As I understand this, I can do this using steps here - Input validation in survey text fields 

Has anyone else done this that can provide some assistance?  Or an Example if done before? My assumption is that this is done in the pre-chat survey html file in Resources, but I'm not 100%.

Thanks, please let me know.