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I handled no less than a 100 calls a month.
I handled no less than a 100 calls per month.
I handled no less than a 100 calls every month.
I handled no less than a 100 calls each month.

Are the emboldened words above can be used interchangeably?
If not, when is each one correct in usage?
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Hi Anon:
These all say the same thing to me.
In conversation, I'd probably use "a month."
In formal writing (a CV, for example), I'd use "every" or perhaps "per."

Regards,
A-Emotion: stars
Comments  
Thanks, AlpheccaStars. This is for a CV. I would use 'every' as you've suggested.