"So why do text messages get through when phone calls can't? For one, SMS text messages are very short, so they require very little capacity when they are transferred over the network. The second reason is that text messaging works by allowing messages to be stored and sent through the network.
If there is a delay in connecting to the network, the phone will store the message in its memory and it will continue attempting to send the message until it gets through. By contrast, voice is a delay-sensitive application. If a sustained connection can't be made, the person on the other end won't be able to understand what you are saying. And so the call cannot be completed."
