system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't my phone make normal phone calls when out of reception, but can make 'emergency calls'?
gpt
Each phone company has to install phone towers around the country to get reception to their customers. When you make a call or send a text, your phone is sending/receiving data from the nearest tower on your network, which will usually be less than 10-20 miles away. The tower then sends to the signal along its way.

When you have no reception, it is because your phone cannot reach your tower. However, there may be other towers from other networks that you can connect to. 

Essentially there is an agreement in place between networks that they will share resources for emergency calls. So if you cannot access your tower, your phone will automatically connect to the nearest tower, regardless of whether or not you are on that towers network.