E.164 is the international standard for writing a phone number so that it is unambiguous anywhere in the world: a plus sign, the country code, then the national number with no spaces, brackets or dashes, up to 15 digits in total. A US number in E.164 looks like +14155550123. Messaging APIs, RCS included, address every recipient this way.
Why it matters for messaging
A number stored as (415) 555-0123 is fine for a person and useless for a system, because the same digits could belong to several countries and the formatting varies by source. E.164 removes the ambiguity, which is why an RCS or SMS API rejects anything else, why capability checks and delivery receipts are keyed on it, and why an opt-in record should store the number in this form from the moment it is collected.
Common mistakes
Leaving off the country code, keeping a leading zero from a national format, or storing the number with formatting characters. A list of numbers imported from a CRM or a spreadsheet usually needs normalising before a first send, and a number that fails E.164 validation is a data problem to fix upstream rather than a message to retry.
On SimplyRCS
The API and the campaign builder accept and return numbers in E.164, and imports are normalised on the way in. The RCS API overview shows the request shape.