how many mobile phones are useful?

Back in the old days of landlines (POTS) I had two phone lines. One with an unlisted secret number, that I only used for outgoing calls and another one, that I gave everybody, for all the incoming calls and the answering machine. Then at the end of 1997 I got a GSM cellphone and got rid of all my landlines. Luckily I could access the Internet via cableTV and later through fixed wireless. Pretty soon I found some good reasons to have more than one mobile phone. Primarily it is redundancy. There are locations all over the world that have great coverage with one provider and a terrible one with another and vice versa. Secondary is cost. Calls in the same network normally are cheaper than outgoing calls to another network. Third is roming capabilities and cost. Some providers are great for calls inside a given country, others are great for roaming purposes, which is especially important for GPRS data access. So since a few years I am carrying now two mobile phones with SIM-cards from different providers and I'm quite happy with them. I would be quite interested to know how many mobiles other people are carrying and why...