How can I make him understand?

So my boyfriend doesn't work in retail and can wear headphones at work. While he is working, we talk almost his whole shift. So I work in retail where we have to communicate a lot and my boyfriend wants me to be on the phone with him while I'm working. He calls me and tries to have a long conversation with me but then I tell him I can't talk long and hang up. He thinks that I'm seeing someone at my job.
How can I make him understand?
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