I had this same question about his cell phone usage, lack of usage, access to towers, etc. I also wanted to know if his laptop remained connected to the HI network through the night or not. I think not. Not sure about his phone. Don't remember any testimony about his phone being turned off or not.
Here's a timeline from Just the Fax from a thread during Trial 1 (JY calls his mom, then leaves MF VM to go by house and directly afterwards calls the Money mistress (having talked to her just before midnight the night before):
http://www.websleuths.com/forums/showthread.php?p=6651919&highlight=cell+phone+ping#post6651919
"This may help....calls he made 11-3-06
++++++++++++++++++++++++++++++++++++++
Calls - Pat Young
28 calls
7:40am Calls Pat Young...First cell phone ping from Jason's phone since 11:42 on 11/2.
From a point SW of Wytheville, VA.
Approximate distance from Hampton Inn: 33 miles.
Calls Meredith Fisher...VM (message to go to house)
7:48am
Calls - Michelle Money
7:49am Jason makes his first call of the day to Michelle Money.
12:02pm Jason calls Michelle's work number.
12:06pm
Calls - Meredith Fisher VM
12:10pm Jason leaves a cell phone message for Meredith asking her to retrieve eBay prinouts from the house.
Calls - Michelle Young
12:17pm Jason calls Michelle's cell phone number.
Calls - Meredith Fisher
1:35pm Jason sends text message to Meredith's phone
Calls - Meredith Fisher
1:36pm Jason calls Meredith; leaves voicemail (45 seconds).
Calls - Pat Young
1:37pm Last call between Jason and Pat
Calls - Linda Fisher
1:53pm 1 call from Linda to Jason at 1:53:04 (0 seconds).
Calls - Linda Fisher
1:53pm 1 call from Linda to Jason at 1:53:23 (44 seconds); logged as a missed call due to Jason speaking to Michelle Money.
Calls - Linda Fisher
2:51pm 1 call from Linda to Jason (0 seconds)
Calls - Linda Fisher
2:52pm 1 call from Linda to Jason (38 seconds)"