If someone is standing ten feet away from the register, talking on the phone, it would not even occur to me that that person was queuing for a register, especially if there was no one actually at the register and the cashier was just sitting there, waiting for the next customer. Had she been standing directly at the conveyor belt, for example, then I would have considered her to be first in line and waited behind her.
I was actually the woman with the phone just the other day. I was at the supermarket and heading for the check-out when my phone rang - DH wanted me to get something for him, so I moved back a bit from the registers, maybe a bit less than ten feet (without blocking anyone), and let him explain what he needed. In meantime, someone else went to the register I had been heading for. That's not cutting in line, to me - I obviously wasn't ready to check out yet, but they were, so they went to the register before me.