Not all call back to get help (3)

I believe the answer is:

aid

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This could be a double definition.

'not all call back' is the first definition.
'aid' can be an answer for 'back' (I've seen this in another clue). I am unsure of the 'not all call' bit.

'help' is the second definition.
(synonyms)

'to get' is the link.

Can you help me to learn more?

(Other definitions for aid that I've seen before include "charity" , "assistant" , "grant" , "Relief" , "Help. Assist" .)

I've seen this clue in The Independent.

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