I've been working in Kenya as a technical Co-founder of a startup in the ag space. We focus on smartphones, but I used to work with the telecom operators a lot in my previous work (where basic GSM connections gives some indication of population connected).
Some of the best stats you get for telecom coverage will come from the annual reports of commercial telecom companies. Eg in Kenya you can look at safaricom. In somalia I guess you can look at Telecom Somalia and some of the others (keeping in mind their market share).
GSMA do research every few years on the number of SIMs on average per market. This is how they get to a figure about subscribers (ie people) rather than connections (number of SIMs). The latter is what is usually reported by commercial companies.
You can quite easily do some online digging for a given market to get fairly current numbers (ie last year). For doing anything with sizing markets etc I'll always start here. Not with World Bank or similar data which is really outdated and not indicative of current trends in technology at all (which is important for me).
The international development sector is unfortunately so slow to realise what's going on technologywise - i'd wager Facebook will know a huge amount more about populations in a lot of these African countries than their governments or the international development sector do. Perhaps if they were a bit more genuinely philanthropy inclined they could do some amazing socially impactful stuff right now. But I don't think any of us will count our chickens on that one.