On the Cape Verde Islands, there are two mobile phone network providers with quite reasonable rates for telephoning (national) and internet:
“CVMovel” and “Unitel / Tmais“.
In most places 4G network is available, otherwise almost everywhere 3G.
Some SIM cards are given to you as a gift on arrival at the airport.
You can then top up the SIM cards and activate various packages, e.g. a 6GB package with CVMovel costs about 10 EUR – valid for 1 month, incl. flat rate to call other mobile numbers of the same provider.
There is the possibility to buy credit in most supermarkets.
For initial activation/configuration, it is best to go to a CVMovel or Unitel shop.