US Cellular since 1996-97. Currently have 4 lines at $172 a month.
I am looking to potentially switch soon.. Shame I can never get T-Mobile's Network Pass to work or I'd probably go there.
Ouch. I'm paying around $240 a month, but that's for eight lines. A small, but very real reason I left Sprint in 2015 was they were charging me close to $200 for four lines. And two of those lines had 1GB capped data!
Granted, my current T-Mob plan is grandfathered (I've been on the same Simple Choice plan since I joined in 2015). The era of T-Mob customers with $50 bills for 10+ lines is over though. In the last couple years T-Mobile had stopped offering LOUs (Lines On Us), which are free line promos, and attaching more conditions to the ones they do offer. Lots of long time customers just kept collecting free lines over the years, which is how you get what I mentioned.
And of course the current plans are going to be more expensive - in comparison to older plans. You might get a good deal compared to your current plan, but if I were to move to the plans being offered right now my pricing would be around $350 or so.