Alarm is silent because "Unable to play during call" because of facebook messenger

This morning, the alarm did not wake me up. When I change the ringtone, I cannot hear how the new ringtone sounds (the phone is still silent) but I get the message > Unable to play during call My conclusion is that the phone did not sound when the alarm went off earlier this morning because the phone was "in a call". An alarm should not be blocked by anything (and there was no call). How do I fix it? I closed apps one after another and when I had closed facebook messenger, I could hear ringtones again. But as long as the phone thinks running a particular app makes it being "in a call" and as long as being in a call blocks the alarm sound, the question remains.