They happen to mean phone went off (9)
I believe the answer is:
phenomena
'they happen' is the definition.
(I've seen this before)
'mean phone went off' is the wordplay.
'went off' indicates an anagram.
'mean'+'phone'='meanphone'
'meanphone' with letters rearranged gives 'PHENOMENA'.
'to' is the link.
(Other definitions for phenomena that I've seen before include "spectacles" , "Observable occurrences" , "Things sensed" , "Unusual developments" , "They happen" .)