They happen to mean phone was out of order (9)
I believe the answer is:
phenomena
'they happen' is the definition.
(I've seen this before)
'mean phone was out of order' is the wordplay.
'mean phone' can be anagrammed to 'PHENOMENA'.
Though, I am not clear how the anagram is indicated.
'to' is the link.
Can you help me to learn more?
(Other definitions for phenomena that I've seen before include "remarkable people" , "Observable events" , "Remarkable events or things, wonders (9)" , "extraordinary things" , "He-man open to remarkable things or wonders" .)