We strongly encourage E.164 number formatting for all phone numbers both in the ‘to’ and ‘from’ fields. This format is the internationally-standardized format for all phone numbers, and it includes all the relevant information to route calls and SMS messages globally. E.164 numbers can have a maximum of fifteen digits and are usually written as follows: [+][country code][subscriber number including area code]. Phone numbers that are not formatted in E.164 may work, but it depends on the phone or handset that is being used as well as the carrier from which the call or SMS is being originated.
For example, to convert a US phone number (415 599 2671) to E.164 format, one would need to add the '+' prefix and the country code (which is 1) in front of the number (+1 415 599 2671). In the UK and many other countries internationally, local dialing requires the addition of a 0 in front of the subscriber number. However, to use E.164 formatting, this 0 must be removed. A number such as 020 7183 8750 in the UK would be formatted as +44 20 7183 8750.