Telephone call that is the best man's responsibility
I believe the answer is:
ring
I believe this is a triple definition!
'telephone' is the first definition.
(I know that telephone can be written as ring)
'call' is the second definition.
(telephone call)
'that is' acts as a link.
'the best man's responsibility' is the third definition.
(I've seen this in another clue)
(Other definitions for ring that I've seen before include "'It's a square, strangely, for boxers (4)'" , "Make sound like phone" , "Buzz" , "sound as a bell" , "Call - cartel" .)