contactEndpoint

The destination of the contact method, such as an email address or a mobile phone number.

Use the E.164 format when specifying a mobile phone number. E.164 is a standard for the phone number structure used for international telecommunication. Phone numbers that follow this format can have a maximum of 15 digits, and they are prefixed with the plus character (+) and the country code. For example, a U.S. phone number in E.164 format would be specified as +1XXX5550100. For more information, see E.164 on Wikipedia.