How are we supposed even to start to answer those questions without knowing what cell phone you are using and to which carrier you subscribe? Better questions get better answers.
Did you check 3G? Did you call its tech service? Some of its antennas/amplifiers re-broadcast (i.e., they are repeaters), which is what I thought you wanted rather than a direct plug in, based on your comment about providing service to people who visit your home.
Let's assume you are Verizon. If you search on Verizon towers, you will get a map if its tower locations and service areas (e.g., 4G LTE extended, 4G LTE, 3G, 3G extended). Couple that with Google Earth and you can find out elevations, obstructions, distance, etc. The best tower may not be the closest, depending on terrain.
As for an antenna jack, they are very small. Not all phones have them. On the Verizon JetPack I used, the jack was quite inconspicuous.
And finally, 3G Store (probably others too) will tell you how to do a "site survey." The JetPack I had and many cell phones have aps that will allow you to walk around and measure signal strength. So, if you get a directional antenna and repeater, then you set your phone in range of the repeater, dial up the signal-strength ap, and start testing with the antenna.
John
Edit:
Here are some links that may help you. The easy way is to just skip to the last one.
https://support.verizonwireless.com/clc/devices/knowledge_base.html?id=16455
**broken link removed**
**broken link removed**
https://forum.xda-developers.com/showthread.php?t=1800798 (shows signal strength measurement)
Or, you could just try this(be sure volume is up):
https://www.youtube.com/watch?v=UVE2x65MnN4