Yeah, unfortunately it's pretty arbitrary and you have to learn as you go... But it seems that for most 'online' or technology communication devices, when we are doing something we say we are "on". For example, "Are you on Facebook?" "Shhhh be quiet I'm on the phone!" "I'm chatting on WhatsApp", etc.
If you stored a phone number, though, you would say "I have your number in my cell." But if I did the action and called you, I'd say "I'll call you on your cell."