@Lost Boy
Congestion doesnt effect your signal. Congestion effects what you can send and receive depending on the traffic on the network.
When people say the network is down or whatever, it doesnt really mean the network has crashed, its simply that everyone in the Uk is sending/receiveing calls and texts through the network and it reaches full capacity. Once at full capacity it wont let any more texts or calls thru until theres enough space for your text or call.
However, the cells have a priority to 999 calls, so if you make one, it would drop a "personal call" in order to let yours through. No matter how congested the network gets, an emergency call will take priority.
Does that make it any clearer? or have i confused you more? hehe
Happy New Year!!