May help. They had me turn Find My iPhone off, hook the phone to a computer , and wipe it out and reinstall iOS completely. Remember to have iCloud on for photos and everything else you want back on the phone before you wipe it out (contacts, mail, etc). Then, when you set up the phone anew, just log in with your Apple ID and don’t restore from back up during the process. iCloud should repopulate everything but your apps, which you can install over time by redos losing them one by one.
So far, my 11 has performed 99% better since doing this. Still notice attic every now and then, but not as often, and the biggest improvement has been fewer dropped calls. It still happens but not 6 times on a single call. It may just happen once a day now. My 6s by comparison never gave me a single problem.