Airside at airfields which make it a condition of contract that phones be switched off, I switch my phone off.
I didn't know that. Where is this written? Contract between exactly who?
It would make it just slightly difficult to get SMS messages from homebriefing.com advising me of slots, etc. Or to phone the tower to ask if they have my flight plan (which I have filed from my internet-enabled laptop while sitting in the cockpit on the ground - this laptop uses a GSM radio just like a phone but a bit more powerful).
Loads of exec jets have a satellite phone installed.
Etc.