Crossword Genius

Jointly endorse function over the phone (6)

Ross

I believe the answer is:

cosign

'jointly endorse' is the definition.
(I know that to cosign is to jointly endorse)

'function over the phone' is the wordplay.
'function' becomes 'cosine' (mathematical function).
'over the phone' shows a homophone (sound like) (how it might sound over the phone).
'cosine' sounds like 'COSIGN'.

I've seen this clue in The Times.
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