Signal

The signal coverage that users can search for and see on various signal strength urls are just INDICATORS, they are not indicative that the user would get a usable continuous signal at all times. Think of the MPG thats stated for your  car's spec, its more than likely done in lab with the figures derived from the output direct from the engine rather than from the wheels with no wind/turbulence etc..... looks good on paper but you are unlikely to achieve that in 99% in real life.

Trying in different rooms in your house and see if it makes a difference and yes I know what you're going to say before you say it, you shouldn't have to move to a particular place/room in your home just to receive a signal and hence a call. I already know  what their customer service would say if I contact them with the same issue.....