It's nothing to do with Germany specifically, but travelling abroad without activating the free roaming facility means a phone won't connect to foreign networks and consequently not have coverage outside the UK.
It's not a phone setting but a network one - on pay-monthly it's normally checked by texting ROAMING to 150 before leaving the UK. The £2.29 billing add-on is just that - a billing add-on, and is separate from enabling the roaming facility which is free.
If the phone is abroad and roaming is enabled, then connecting to alternative local networks usually resolves. If roaming is not enabled, then then CS will need to activate it.
