The POS should not require to enter the country code to create a customer account, the country code is normally not required, and nobody will ever tell his number including the country code, so it makes easy for any employee to forget to enter it and if that happens it creates problem in the POS to send, receive and recognize incoming SMS. I pretty sure most if not all POS user do only local business and they don't need to contact international customers, so the POS should not require the country code. And if necessary the POS may have the option to change the country code but I think it is unnecessary.
This had caused me a lot of problems as when that happens I don't get the incoming SMS in the same tread under the ticket number.