If they don't answer they are probably in the shop doing something for another customer. I've been at the register and they have answered the phone so you probably just caught them at the wrong time.
I generally just go in as I am out and about (driving for Uber) and find out if they have time then, or do I need an appointment. Actually, I don't recall ever calling them, but they usually are staffed. They might be away from their desk/station (perhaps getting a drink from the vending machine). If I recall, the last time I was there, I found that their next spot open was a Sunday, so I went there the following day and it took like an hour or hour and half.
They are supposed to answer within 4 rings.
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