Formatting Mobile Numbers
We strongly encourage E.164 phone number formatting for all phone numbers. This is an internationally recognized standard phone number format that will help to ensure deliverability of SMS messages across the globe.
E.164 Phone Number Formatting
E.164 formatting for phone numbers entails the following:
A + (plus) sign
International country calling code
Local area code
Local phone number
For example, here's a US based number in standard local formatting: (415) 666-1234
Here's the same phone number in E.164 formatting: +14156661234
In the UK, and many other countries internationally, local dialing may require the addition of a 0 in front of the subscriber number. With E.164 formatting, this 0 must usually be removed.
For example, here's a UK based number in standard local formatting: 020 1234 5678
Here's the same phone number in E.164 formatting: +442012345678
If you are unsure of the correct Country code, a good reference is this Wikipedia page that lists countries and their calling codes.