Jean, you'll be able to get a SIM card for your phone at the airport on arrival in Nairobi. Just take it into the Safaricom shop on the right, in the concourse, as you come out of the arrivals hall and they will fix you up - assuming your phone isn't locked to a UK network - check. Very helpful people at Safaricom. It'll cost you about £1 for the SIM and a little bit of airtime. Then you just buy whatever you need in cards of Ksh100, Ksh500, Ksh1000 etc. Just like here. The airtime goes a long way! You'll get 3G, again just like here, all over Nairobi, in the main towns, often along the highways, and even out in the sticks, so you can use your phone as normal, except of course you'll have to remember to tell friends and family your temporary number.
If you're locked to your UK network, and can't unlock before you go, then you can still use your iPhone in all the normal ways, but you'll be on Roaming charges from you're UK provider, and it will cost you a bomb, so be sure to turn off 3G when you don't need it. And keep a check on your Network Usage!
Everyone in Ortum and around Pokot country will know all the best spots for getting network. There's usually someone nearby in really rural areas who has a big battery and can charge phones for people - nice little earner if you can afford the battery in the first place!
Have a great trip. The Cheranganis are beautiful.
Edited: 2:54 pm, November 21, 2012