It depends what you mean "cell tower". All the carriers have very inexpensive microcell device. Some are placed say in parking lot of a large business park. They are small boxes you see on a lamp post. Still the box is not the costly part it is the connection back to the network and they would either need to run microwave backhaul or pay for a circuit.
Now if you just have crappy cell coverage in your house or business you can get even smaller cell devices that are no bigger than a modem. It uses YOUR internet to build a VPN tunnel back to the provider. It also needs to at least get some signal from GPS satellites to keeps it clocking correct.
These sometime you can get for free if you are putting it in a business or they will sell them to you for a couple hundred dollars. You of course only want to make voice calls on this system. They do have LTE support but that would be very silly since you are now using your LTE quota from the cell provider to get to the internet on a connection you already pay for internet directly. They are more than happy to allow you do this since other than a vpn connection there is no day to day cost the cell provider is incurring. It is very low power it does not go much past your house but you will get 5 bars of coverage inside the house.
You would think I would have learned. This is a old thread that I got baited into posting on. The technology is very different now than 3 or 4 years ago.