A friend has tried to call me couple of times but has come to the voice mail every time. SMS didn’t work either. Called the operator but no luck there. Has also tried to restart the phone a couple of times but that didn’t help.
Finally this morning I found out what setting what was causing this problem. It turned out to be “Call Waiting”. In a Samsung Galaxy S6 phone this setting is found in Settings > Applications > Phone / Telephone > More settings > Call waiting. Turned this one off and then it started to work.