Is it a fault in a mobile service if I have no coverage?

It can be. A service should correspond to what you and the operator have agreed upon. If the coverage is worse than what you were promised, then there is a fault with the service. Mobile services can be affected by various factors such as capacity in the mast, and the services are usually described in the terms of agreement in a way that makes it difficult for consumers to show that poor coverage in a certain location means there is a fault with the service. In, for example, ARN 2014-09029, the consumer had not proven that his lack of speed on mobile broadband was a fault in the service in relation to what was agreed with the operator.