Why is my phone glitching?

So I haven’t had this phone over a year and it’s paid off, everything was fine at first. After the last 15th update things started to go bad. When I answer group calls it kicks everyone out and hang up. Aside from that since I updated it to the 16th update it has now started to hang up calls when some else calls! Get this my messages say 2 different numbers I can’t tell how many messages I have! Sometimes it kicks me right off the app I’m currently on and starts to lag. I don’t know what it is but it has to be fixed!!