Can an airline crew detect if a cell phone is left on?

No. The requirement to have them turned off from before takeoff to after landing is an archaic rule. They don’t work anyway once airborne, and once the groundspeed exceeds a certain value, the cell towers don’t process their signals. Also, Cell towers are designed to have a horizontal beam, not a vertical beam, so the reception capabilities quickly drop below the required minimum signal strength, once above a fairly low altitude.

Here’s a tip to make phone call while airborne. Get on the aircraft’s WiFi. Use the FB messenger app’s phonecall feature to make a crystal clear call to anyone else with FB messenger. Now you can be annoying all of the time!