We moved to Clay County, where Orange Park was good for everyone but our friends with AT&T.

"It's slower for me," said Jennifer Harmon, who has an AT&T Android phone.

"Sprint is holding up," Percina Infantino said as she tested her Sprint iPhone.

Interstate 95 at the World Golf Village exit was a bad spot for a lot of our volunteers.

"No phone, no text message, no Internet," said Wells, who has AT&T.

We found the tourist Mecca of St. Augustine brought mixed results.

"Mine is pretty good here," said Shakena Fagan, who has a T-Mobile Android phone.

Harmon and Lora both reported a poor signal with AT&T.

"My upload speed went to zero," said Lora. "Zero, done, I got nothing. I can't Instagram. I'm depressed, that beautiful picture I just took a San Augustine, I cannot."

Our 4-hour tour taught us that different cellphone carriers have good spots and bad spots in our area. The best performing of the four major carriers was Verizon Wireless. We couldn't find any problems anywhere on our route!

"That's why I pay them the big bucks," said Ted Whittle, who has a Verizon Android phone.

We found no significant difference between service in iPhones and Android devices.

AT&T and T-Mobile were the next best carriers and Sprint was the worst in our test.

"I'm probably not going to stay with Sprint," said Castillo, whose contract is up in a few months. "Because it just doesn't work in Jacksonville."

Just a few days ago, on May 16,  RootMetrics, a company that tests cellphones, released new findings in the Jacksonville area.  It's tests also found that Verizon was the most reliable and the fastest in North Florida. AT&T was a close second. All the carriers got high marks for text message performance, but overall, Sprint lagged behind the rest, just like in our test. (See RootMetrics test results in Jacksonville here.)

"I think some companies just aren't aware how bad the problem is in some areas," said study participant Crawford.

To find out why service can be so good for some and so bad for others, we went to Dr. Brian Kopp, an electrical engineer, communications consultant and University of North Florida adjunct professor.

"The peak times are in the mornings, the start of the business day and in the evening when people are using their phones in their homes maybe, you will see an increase in the number of users on the network and that generally means the per user speeds go down," Kopp explained.

So time of day is a big factor and so are towers. They're placed all over the area. In general, if you're closer to your cell company's tower, you should have better luck. There are also much smaller receivers on buildings that help boost your signal.

"If you are further away, if you are out in a rural location, you may only get the 2G or 3G service," Kopp said. "It is definitely slower then the LTE."