Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
A. your home is outside of their service range, or
B. your home blocks, masks, or jams the signal from your service provider, with radio noise or radio wave shielding.
For A: try stepping outside. If it doesn't work even when you're out in the open in your yard or other open space near your home, then it's outside service range - perhaps because the nearest cell phone relay tower or station for your service provider is behind a mountain or a hill or a large steel-framed builiding.
For B: the previous test, but also, consider whether your home has a steel framework, metal studs in the walls and no windows, or appliances or a possibly defective portable phone (that could be turned Off and unplugged too, to test whether that allows your phone service to work), or other appliances esp. a microwave or an electric heater or air conditioner (again, test by turning them off and unplugging them if that's possible)...
What do you think, given what I've just said?