Vocal practice: feed into the endless calling (6,7)

'vocal' is the definition.
Both the definition and answer are adjectives.
Maybe there's a link between them I don't understand?

'practice feed into the endless calling' is the wordplay.
I cannot really see how this works, but
'into' could be 'in' and 'in' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?