Can a player turn down a Cup of Nations call-up?

A top football lawyer has told us that any footballers who refuse a call-up to the Africa Cup of Nations could get themselves and their clubs into big trouble.
English Premier League players who go to the tournament face missing up to six club games.
And some have already said they won’t go.
7 Cameroon players say they will turn down call-ups including Joel Matip of Liverpool and Allan Nyom from West Bromwich Albion.
David Seligman works with many big names from African football.
He says national teams can force players to go to the Africa Cup of Nations.
BBC.com