BR-KSA-85
The Buyer's contact phone number (BT-57) shall start with "0" or "+", followed by a maximum of 15 number and minimum 4 character after the "+" or "0" , if exist.
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
//cac:AccountingCustomerParty/cac:Party/cac:Contact
matches(//cac:AccountingCustomerParty/cac:Party/cac:Contact/cbc:Telephone, '^(?:0|\+)[0-9]{4,15}$')
Checked with Saudi Arabia ZATCA Tax Invoice (UBL), Saudi Arabia ZATCA Simplified Tax Invoice (UBL)