There are a couple of options. 

If your phone supports WiFi calling and your provider allows it, I strongly recommend getting WiFi in the shack if you don’t have it already. If you don’t have it, and that’s because you’re remote, get a LTE<->WiFi router with SMA connectors and an appropriate antenna.

If you can’t use this solution you can buy cell phone boosters on amazon. Get one that has multi band support, the one I have is five band and that works. Get the model that comes with an appropriate antenna, if you’re anywhere near a tower an Omni will work fine. 

The downside to the booster is that you can get feedback between the interior and exterior antenna, which in my case rendered it useless- the cell tower was so far that any level of boost led to the local signal being strong enough to cause feedback. If you have a strong signal, combined with your foil shielding, I suspect it will work perfectly.