Mandate '+' in all Phone Number formats

**Problem description**
The standards are a bit iffy around whether or not a '+' is required for phone numbers, we've agreed in SimSwap to have a '+' only, but I think it makes its sense to make it an agreed CAMARA standard across all APIs.

**Possible evolution**
https://github.com/camaraproject/NumberVerification/issues/65