What’s the line between professional and flirting?

Ok so at the new role I went into we are mainly on the phones and in my training class we have begun to take calls in real time to essentially get down what we are gonna be doing. Well anyways I was on the fence about practicing but then two other girls said something that I wasn’t sure how to take they said they liked how my voice sounded on the phone cause we also listen to each others calls. I’ll be honest I blushed because it’s one thing to compliment how someone did on the call but to say you like how their voice is on the phone and to encourage them because you want to hear their voice

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Really curious
What’s the line between professional and flirting?
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