Cape Verde is classed as outside the EU so you will need the O2 Travel Bolt on. It will cost you £6.00 per day (when used) Text O2TRAVEL to 23336 at least 24 hrs before you go.
Use Wifi where possible so you reduce the need for O2 Travel. A simple text will trigger charges for O2 Travel. Turn Voicemail Off. Stay in touch with friends and family by using apps like Messenger and Whatsapp over Wifi. Make sure they have these apps installed.
Enjoy your holiday.
Veritas Numquam Perit