Cover two extensions from one phone

A handset can now register a second SIP account on its own line key.

Each line is an independent registration — not a shared-line appearance or a call-forward — so both extensions are individually addressable and individually reachable.

Per-line behaviour

• Independent registration. Each account maintains its own SIP registration and presence state.
• Dedicated line key with its own lamp state for idle, ringing, and in-use.
• Outbound caller ID follows the selected line, so calls placed from the second account present that extension's identity.
• Configured per device, not per model — two identical handsets can carry entirely different account bindings.

Supported on Grandstream handsets with an available line key. Line-key capacity varies by model.