Hello,
Neither Stripe nor Readymag were able to assist me with this issue.
So, I am reaching out to the community for help, thank you in advance 
I need to collect my customers’ phone numbers, but this is not directly configured in the Stripe x Readymag setup.
There is a very simple tutorial to customize the Stripe code in your CMS: Collect customer phone numbers with Checkout | Stripe Documentation
The only option would be to implement it via Custom code in Readymag. Since I lack the skills to do this, perhaps a developer can assist me?
A big thank you in advance, I believe this will help many others too!
Have a great day,
Corentin