Phone number country prefix in ShippingAddress

Hello,
I’m trying to get the full phone number from a shipping address using GraphQL API.
I have an example of an address with a phone number like “123456789” with no country prefix via API. I was expecting “+XX 123456789”.
At first, I thought about using the address country code to guess it, but when creating an address, a customer can type an address in a country and choose another country for the phone number. I did not find any way to get the country for the phone number with the API.
Has anyone managed to get the full phone number in all cases ?
Thanks

The phone number should be the full one entered at checkout.
What does your checkout look like for phone number entry? Have you done anything custom here?