Using cell phones in Jets at high elevations

I remember back between 1986 and 1996 I used to fly a lot across the country and my cell phone was able to make calls. It would cut out from time to time but I simply called back the person that was dropped.

The interesting thing was they were never on my bill. I used to be curious where I was when I made the call and it never reported on the bill.

I’m talking about a jet at flight levels above 38.

I’ve tried it in the last 6 years and don’t think I’ve been able to make a call up there with a cell phone.

My question is why was I able to make calls then but now now? What has changed?

There are a lot of people who use this argument that people could not make calls on their cell phones from the jets during 911.

If you know this answer, would you know why the calls never made it on my bill either?