RexaThe short answer
E.164 is the only unambiguous way to write a phone number. A number stored as "(415) 555-0142" is meaningless outside one country; the same number as +14155550142 routes from anywhere. Voice APIs standardise on it because normalising at the boundary eliminates an entire class of routing bugs and duplicate-contact errors.
In detail
An E.164 number is a plus sign followed by a country calling code and a national significant number, with a maximum of fifteen digits in total and no spaces, hyphens or parentheses. The plus is part of the format and signals that what follows is international rather than dialled through a local prefix.
The standard is a format, not a validity check. A string can be perfectly well-formed E.164 and still be unassigned, out of service, or not the type of line it appears to be. Confirming that a number is reachable, or that it is a mobile rather than a landline, requires a lookup against carrier data rather than a regular expression.
Normalising to E.164 on input is what makes deduplication, suppression lists and consent records reliable. A do-not-call entry stored in one format will not match the same number stored in another, and that mismatch is a compliance failure rather than a data-quality annoyance. The same normalisation underpins area-code-based timezone inference for calling-window enforcement.
How Rexa handles it
Rexa normalises to E.164 across the API surface: origin and destination numbers, transfer targets and suppression entries all use it. The area code of a normalised +1 number is also what drives recipient-local timezone inference when a contact record carries no explicit timezone, so calling-window enforcement depends on the number being stored in E.164 form.
Calls APIRelated terms
DID
A DID is a telephone number assigned to an organisation that routes an inbound call directly to a specific destination without passing through an operator.
Do Not Call
Do Not Call refers to the registries and internal lists of numbers that must be suppressed from telemarketing contact.
Calling window
A calling window is the range of local hours during which it is permissible to place a call to a recipient, with the hours outside it known as quiet hours.
PSTN
The PSTN is the global interconnected system of circuit-switched and packet-switched telephone networks that carries ordinary phone calls.