E.164 Phone Number Format In Checkout

Site URL: https://alfiescoffeeco.co.uk

I'm setting up automation for my ecommerce site where by when a customer places an order they receive an sms message as well as the standard SquareSpace email.  The service I am using requires the phone number in E.164 format.  Is there anyway of making this format the standard on my checkout page? For example, rather than the telephone number being 07712 345 678 it is +44712 345 678.

Thanks!