A short code is a 5- or 6-digit number that can send and receive SMS (and MMS) to and from mobile phones. Twilio currently offers short codes in the US, Canada, and the UK.
Short codes come in two varieties: vanity and random. A vanity short code is a specific number that you choose, while a random short code is assigned to you and can’t be reviewed in advance.
Short codes must be leased from a national registry for a minimum of three months at a time. Twilio will lease a short code on your behalf from this registry, unless you prefer otherwise.
Twilio short codes can send 100 messages per second by default; this limit can be raised for an additional fee. Because short codes can send high volumes of messages, wireless carriers individually approve every short code for its intended use. This approval process typically takes 8 to 12 weeks.