You travel to another country. Your phone still works. That is roaming — using a mobile phone outside the home network area. Your home carrier has agreements with foreign carriers. They let you use their network. Your home carrier bills you. Roaming is convenient. It is also expensive.

Roaming works through interconnection agreements. Carriers exchange signaling and billing data. When you register on a foreign network, your home carrier is notified. It authorizes the service. It tracks your usage. It bills you later. The foreign carrier charges your home carrier. Your home carrier marks it up.

Roaming types

• National: outside home region, inside country
• International: outside home country
• Data: internet usage
• Voice: calls
• SMS: text messages

Roaming charges have fallen. The EU abolished roaming fees in 2017. Other regions have not. Travelers still face bill shock. A few minutes of video can cost hundreds. Tips: buy a local SIM, use Wi-Fi, or get an international plan. Roaming is convenient. It is not cheap.