SIP registration explained

Your phone cannot just sit there and expect calls to find it. Before it can receive a single call, it has to actively tell the network exactly where it lives right now, and it has to keep saying so, over and over, for as long as it wants to stay reachable.

An address book entry that expires

Every SIP account has a permanent identity, called an address of record, something like sip:1234@yourpbx.com. That identity never changes. But the actual device answering it, your desk phone, your softphone, your mobile app, changes constantly as you move between networks or restart the app.

REGISTER is the message that binds the two together. It tells the server "this permanent address currently lives at this specific device address." Without it, the server has an identity with nowhere to actually deliver a call.

Why it has to keep happening

A registration is not permanent. It comes with an expiry, often somewhere between five and ten minutes, and the phone has to send a fresh REGISTER before that timer runs out or the server quietly forgets where it lives. This is deliberate: if a phone loses power or its connection without ever telling anyone, the old registration expires on its own instead of leaving calls being sent into a void forever.

Why registration is usually where auth lives

Because REGISTER is the very first thing a device does, it is also the natural place to check whether the device is actually allowed on the network at all. This is why a 401 or 407 challenge, covered in the common SIP errors page, shows up here constantly and is completely expected, while seeing it on every single call afterward usually means something is misconfigured.