Can you call the emergency services with no signal?

I have an iphone and there's a button where you can call emergency numbers like 999. Can I do this with no signal?
I live one street away from the coast and signal is very bad but was wondering If there was ever a time I neede to call 999 could I?

Yes, 999 calls will pick up other network signals to make the call

You need signal but you can use networks other than your own for emergency calls that's what it means when you have no signal but it says "emergency calls only"

Yes - emergency calls allow phones to roam onto other networks which are otherwise locked to you.