What is a SIP-URI?

A SIP URI is the SIP addressing schema to call another person via SIP. In other words, a SIP URI is a user’s SIP phone number. The SIP URI resembles an e-mail address and is written in the following format:

SIP URI = sip:x@y:Port
Where x=Username and y=host (domain or IP)

Examples:
sip:joe.bloggs@212.123.1.213
sip:support@phonesystem.3cx.com
sip:22444032@phonesystem.3cx.com

The SIP URI standard has been defined in the RFC 3261 standard. 3CX Phone System for Windows uses SIP URIs

Links:
RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax.