How does the registration process work when a mobile device enters a visited network?

The device registers with the visited network's mobility manager (foreign agent), which then notifies the home network's HSS so both sides know the device's current location.

* Visited-network registration via the foreign agent to the home HSS. *

Step-by-step:

1. The mobile device enters a visited network and detects a new base station
2. It contacts the mobility manager (foreign agent) in the visited network
3. The foreign agent registers the device's current location with the home network's HSS (the home-side subscriber database)

Result:

• The mobility manager in the home network now knows where the device is
• The HSS in the home network has an up-to-date record of the device's location
• Traffic can now be routed to the device

This registration happens automatically every time your phone connects to a new network — for example, when you land in another country and your phone picks up a local carrier through a roaming agreement.

Go deeper:

• Roaming (Wikipedia) — the live location update where the visited network contacts the home network to authorize and register the device.
• Mobility management (Wikipedia) — the location-update / TMSI registration signaling that keeps the home network's record current.