Mobile phone signals work off of radio waves. If you can block or scramble radio waves, then a mobile phone simply will not work in a given area. Magnets can create effective shielding for the radio waves that make up mobile phone signals. While there are a lot of products on the market for blocking cell phone signals, only some of them work off magnets, and those products are meant for shielding rooms and homes.
Order magnetic wood panels that fit in your home. These panels can be made of a variety of materials, but generally they're made of ferrite sandwiched between wood sheets. This panel has been shown to reduce or eliminate the ability for radio waves to penetrate a location.
Install these panels in your home. Some lighter panels may be hung from the walls or put up with adhesive, or you could install the panels by screwing them into the studs in your walls. The instructions will vary based on the type of panels that you get.
Test your panels by attempting to make a mobile phone call from the room. Alternatively, you could attempt to tune a radio in the room to try and pick up signal.
Combining magnetic panels with other things that block radio waves, such as certain types of paint, can help you get even better results.