Why am I getting a "no signal" message all the time?

I have an account with AT&T that I've had for over 20 years. I've had my iPhone 7 for years. Lately I get a "no signal" message all the time when trying to make calls. This happens even in our fairly urban area of Oakland CA. According to AT&T's web site this phone should work with their network. I tried reaching their support and didn't get anywhere.