On the checkout page, you will be asked to enter your full phone number including dialling code.
The phone number section is made up of 2 boxes
In the first box, the country dialling code should be pre-populated. For example, the UK is +44.
In the second box, you should enter the rest of your number including the area code. For example 01417745023 or 07xxxxxxxxx
This should now allow you to continue and place your order.