I've been using my OnePlus X since 2015. Most of the time I haven't actually had cell service on it, I just use it as a small wifi tablet. If I NEED mobile service, I pay for a month or two of ATT prepaid here and there. But usually I leave it at home and don't carry a phone with me.
Eventually I'll switch over to my husband's secondary iPhone SE, because he wants me to have an actual phone that he can call me on, and it is VERY pocket-sized. I'm not wild about Apple products or iOS, but for something I can stick in my pocket and go, I really only need it to make phone calls, and it does that just fine.