What to look out for when buying a pre-pay SIM card in Africa

This is the fourth time I have bought a SIM card, and my experiences
have varied. There is never a problem with calls, but access to mobile
data has been more complicated, although I have managed to get it in
the last two countries, wherever there has been mobile reception. So I
thought I'd write down some considerations, questions to ask and tips
to remember for those of you reading this who are wont to buy foreign
pre-pay SIMS:

Top tip:
Try to buy a SIM in a cell phone shop. The staff are more likely to
know how to get it working and how to fix problems. That's not always
possible when you're crossing a border, as SIM cards are sold in the
oddest places round here - today I bought my Zambian SIM in a general
trader from an Indian proprietor who was about a hundred years old and
had no clue when I asked about internet on the phone. That means
calling the help desk. Don't expect to get things sorted on the first
call, between accents and terminology it takes a while to get
everything straight.

Questions to ask when calling the help desk:
In order to get mobile data do I have to get the number activated for
it (like on Vodacom SA, Botswana Telcom and Zambian Zain networks -
although not on Orange Botswana, which sets up data access by
default)? Do I need to buy data bundles or does the data come off the
pre-pay credit? What is the APN to access the data network (much
easier to figure out how to set this up yourself than waiting for the
SMS settings that never arrive - one reason I didn't get data in
Namibia was that the text I sent to MTC obviously went down a rabbit
hole)?

Pre-pay SIM cards are cheap, and super useful for making calls in the
country - we are booking all our accommodation a day in advance now,
rather than making a detailed plan. It's possible to buy credit
absolutely everywhere, from clothes shops to general stores to falling-
down shacks in the middle of no-where to women sitting by the side of
the road with little tables selling single sweets and small
denomination credit scratch cards.

Reception has been pretty good, too, considering we have driven
through some seriously remote places. In Namibia there was reception
only in the towns, or near the National Park entrances. In Botswana we
had mobile phone access along all the mayor roads, and we had Edge in
Maun and Kasane. Here in Zambia we have Edge most places, although
when the mobile signal drops, so does Edge, obviously. On the way from
Livingstone to Lusaka, that has only happened a few times.

So all in all it is an unexpected treat to be able to facebook on the
phone and read my feeds while travelling, but it is possible so far.
Who knows how long it lasts.