OK, here is what you need to know. First of all, the lady downstairs.....you're both probably on cordless phones and you're simply picking up her phonecalls.....Your lines are NOT crossed, it's just one of those things that happen with cordless phones. You will sometimes be on the same frequency as someone else who lives near you (this happens a lot more often in apartments where people live in very close proximity to each other.)
That is all that is. You're just on the same frequency or channel. Just change your channel next time this happens OR use your corded phone to call out.
NOW, as for the three-way calling. What a lot of people found out when three-way calling became available to everyone (it used to only be available if you subscribed to is, like repeat dialing, etc.). Anyway, what happened when three-way calling was made available to everyone, people accidentally were making three-way calls and weren't intending to do it and didn't even KNOW they had done it. How this happens and what is happening, I am CERTAIN, with your daughter is this: When she hangs up from a call, she will probably just push the "switch hook" down quickly and dial another number BEFORE the first call has had time to disconnect. When you think about it, this is actually HOW you make a three-way call. You are on the phone with an active call, you press the switch hook down quickly and you get a dial tone and you dial the second number and when you are connected to the second caller, you press the switch hook down again and you're all three connected.
OK, while this is a great service, many people are not disconnecting the FIRST CALL because they're not allowing enough time for the first call to disconnect before they get that second dial tone. Of course, they don't KNOW that it's a SECOND dial tone, they just figure it's a regular dial tone that you get once you hang up and pick the receiver back up again to make another phonecall.
You should call the phone company on Monday to tell them that this is what has happened and they will be able to tell that your number has made three-way calls BUT they know that this is a problem and they will be able to see from your call records that this is what has happened. It takes no special knowledge to do this, because it happens accidentally ALL THE TIME. Just remember to tell your daughter that when she hangs up from her phone call to wait about 30 seconds to be safe and THEN pick up the receiver and make another call THEN. This 30 seconds should allow the first call to completely disconnect.
Hope this helps!!!
A practiced phone person!! If you have a que about any service provided by any of the "Baby Bells", email me. I understand and use them all and I know all their pitfalls and how to avoid them.