Phoning internationally from Namibia is expensive, and it is normally cheaper to get people to phone you back. The international dialing code for Namibia is 264. To dial out from Namibia the international access code is 00.
The cellphone network is rapidly expanding and most towns & villages have coverage. Along major roads cell phone coverage is usually pretty good, but in more remote areas there may be no coverage.
It is possible to buy a 'pay as you go' sim card package on arrival in Namibia (if you are clever and use the services of the Cardboard Box Travel Shop to arrange your self drive safari around Namibia you will receive a free sim card, and other goodies, in your info pack when you arrive in the country). This card can be inserted into your own phone allowing you to make calls at the local rate. It is also possible to rent cell & satellite phones at the Windhoek International Airport.
The Namibian telephone directory includes the countries yellow pages (the yellow section at the back) and a separate section for government departments (the green section about a third of the way through). Towns are listed alphabetically starting with W for Windhoek but then resuming a more logical order.
Larger guest houses & hotels in Namibia generally have in-room phones however the rates can be exorbitant so it is worth checking before making any long international calls.
"If the person you are talking to doesn't appear to be listening, be patient. It may simply be that he has a small piece of fluff in his ear." Pooh's Little Instruction Book.