Pro-700 question - hang up first line when 2nd rings?

Ok, I've hit a rather interesting dilemma with my new Pro-700. When I'm on a call and another call rings in on line 2, if I switch to line 2, one never hangs up until I end the call on line 2, and if I hit end, both calls drop, and if I hit back, one hangs on even if I didn't answer it.

So tell me, what exactly do I need to do to hang up the second line (or the first, depending) when answering or receiving a second call? Especially if one of them hangs up, as it's messing up my phone bill.

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I take it that nobody knows?