I live in the Tulsa, Oklahoma area, and I travel to Arkansas a lot along US-412 and other routes, I-40.....
I ride a motorcycle and I go off the main roads a lot, and can find a lot of places the cell towers don't reach.
but, I will make a comment on "back pocket". You could not find a worse place to keep your phone.
When you sit on it, you are creating the "Faraday Shield" that blocks the cell towers.
That is one of the reasons I carry my phone in my shirt pockets. It keeps the phone up higher, and usually in a good clean environment.
https://en.wikipedia.org/wiki/Faraday_cage
When I am in my home, I can usually get a 4g signal, but usually it is only 1 bar...
then it slips to 3g and 4 or 5 bars... sometimes, I only see 1x....
the areas we use our cellphones in are anything but friendly to radio reception.
While typing this, I am watching my S5 showing 4g and the bars are floating up/dn from 0 to 3...
I can be sitting on my bed and it goes 4g, to 3g to 1x then x for no reason
Actually, there is a reason, you just aren't aware of what it is...
when that happens, you are in an area that does not have a good line of sight to the cell tower(s).
a cell tower might get loaded up with too many conversations and your phone is not in use at the time, so it gets shuttled off to a different tower which is weaker, but still adequate to keep the phone in service to receive a call.
Once you connect, the phone will then attempt to get the best signal it can, and it will switch w/o you even knowing it happened. This technology is highly complex, and we cannot just say that "I am sitting in the same place, all the time, and it should never deviate from 4g".... sorry, but that just is NOT the case.
Things happen.
An airplane flies between you and the tower, you loose the signal just for an instant, and the phone senses that and switches to a new tower.... it will stay there until there is a reason for it to switch again. That reason might be you moving around in the room, someone else moving about the house, or it might be the cell tower "service loads" exceeding its' ability to keep your phone connected to "that tower"...
You say you have had two different phones now, and the SIM card has been changed....
that to me, as a life long Communications Person, tells me, you are in a poor signal area....