Urgent Problem - Phone Number Not Being Accepted

We are trying to book an appointment for a contact, and the booking form keeps rejecting the phone number.

It’s a UK number +44 1446 500720

I select the UK flag for the country drop-down menu, and remove the +44 so it shows 1446 500720, but it still doesn’t work.

The phone number is valid - our call setter reached the contact using that number.
I also used another online validation tool to check the number, it said it is valid.

What can we do in order to complete the booking ASAP?

FYI - I have worked around this issue so I could complete the booking; I simply turned off the Required checkbox, and then did NOT include the phone number in the form; it went through OK. This is not a good solution, but at least I got the current booking accepted. I have now turned back on the Required checkbox.