Crossword Genius

I'm calling up troops to fight and phone round island reserve to start with (9)

Ross

I believe the answer is:

mobiliser

I'm a little stuck... Click here to teach me more about this clue!

'start with' is the definition.
The definition suggests a singular noun which matches the answer.

'i'm calling up troops to fight and phone round island reserve' is the wordplay.
I cannot quite see how this works, but
'up' could be 'is' (I've seen this in other clues) and 'is' is present in the answer.
'phone' could be 'mobile' (mobile phone) and 'mobile' is found within the leftover letters.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

'to' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.
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