Way to test phone service while phone not on carrier?

I have an iPhone 6 that has been having phone signal issues ever since I replaced the charge/headphone jack assembly. iFixit determined I had a bad part and so sent me a second replacement, which had the same problems, and now has sent me a third and I'm still having the problems. Either they've got quite a batch of bad parts, or there's something else wrong. I've been without reliable phone service for over three weeks now.

I have an older phone I could switch my phone service to while I continue to try to fix the iPhone 6, but as I'm trying to troubleshoot phone connectivity issues switching the service away from the iPhone 6 would presumably leave me with no way to test it. I can't just go with reported signal strength as the iPhone 6 will sometimes show 3 or 4 bars of LTE service and yet be unable to make a call. Is there a way I could test the phone's signal connection without having service (this is a Verizon phone)?