Not everyone has that luxury. There are people who have to be on call - sometimes even after hours. Their hellphone numbers are public knowledge - usually included in their sig files - and they're paid a hellphone allowance, and occasionally even a stand-by allowance. These people are few, and typically supervise mission critical systems like the Red Button in the White House, or the SAP system at UCT.
But sometimes people get confused. They think that if you have a hellphone, they have a right to expect that you are simply waiting for their call, and that whatever you're doing cannot possibly be as important as whatever it is crossing their mind, just then. And so, they dial, and leave grumpy messages on your voicemail - grumpier, with each call. The possibility simply never occurs to them that they're using a system designed not to work as they intend.
But now, because some arb UCT staff has my hellphone number, and allowed it to trickle into the public domain, I'll have to retire it. I have two other South African SIMS, and one UK one which is fully operational here, which I could use as my primary SIM instead. Numbers which I would give to three people only - my spawn, my Lover, and my line manager. After all, what else could possibly be that urgent?