If your iPhone writes SOS in the top right corner, you are not connected to your Carrier. To be able to send iMessages from your phone number, you need to be connected to your carrier. This is probably also the reason why iMessages disconnects, as the service cannot register your phone number.
Try restarting your iPhone. When your iPhone turns on, remember to enter your SIM card's PIN code. Without the PIN, your iPhone will not connect to your carrier's network.
If you continue to experience problems, you must contact your Carrier.
You can find more useful information if you see SOS or No Signal here:
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
You can find more useful information here:
If you can't send or receive messages on your iPhone or iPad - Apple Support