My DH sometimes tells the story of how, before I met him, MIL reported him missing to his military base because she hadn't been able to reach him on the phone.
He was (and still is) a serving member of the armed forces but because he wasn't instantly available to answer his phone in the middle of a working day, she reported him missing.
He got into a lot of trouble at work because of her calls to the guard room saying she hadn't heard from him in over a week and was very worried about him (she had been on holiday so it was her out of contact, not him). They sent the military police to look for him.
She said she had made a number of calls to him, but she is the type to call every ten minutes on every number until she reaches you. She rang me 17 times in one day once, over the space of a couple of hours. So her "number of calls" all took place in the same couple of hours on the same afternoon.
And once when she knew I was out (DH had told her earlier) she rang my Mum to say how worried she was that I wasn't answering the phone at home and how she thought something terrible must have happened to me. My Mum was 250 miles away (as were PILS) and would have been really worried but luckily I had spoken to her earlier and told her I would be out.
With my MIL, it's about control. If she cannot speak to you the second she decides she wants to she is offended and slighted and FIL panders to her in this. Before we stopped seeing them, if they left messages on our machine they went like this.
MIL: Oh...it's your machine, I'll try your other phone.
MIL: It's me, I can't reach you...call me back please.
MIL: It's me, you haven't called me back.
MIL: You still haven't called me back.
MIL: Is something wrong, why aren't you calling me back?
MIL: Have we done something? Why are you ignoring me?
FIL: Your Mother is in tears, call her back.
MIL: I'm sorry about your Dad, I just don't know what we've done to upset you. Why won't you call us back?
FIL: CALL YOUR MOTHER!
MIL:
DH's Brother: You think you are too fucking good for us don't you?
MIL: Where are you? Something's happened hasn't it? You're hurt aren't you? I'm going to call the hospital in a minute. Something must have happened to make you ignore your phones? Where are you? Call me back!
DH's other brother: Just had Mum on the phone, ring her back mate.
MIL: You've had a car crash haven't you? Or someone is ill? We are going to drive down to see you!
And they went on like this until we got home from work and realised the machine had been filled with this nonsense.
Then DH would be in a foul mood because he knew if he didn't ring it would carry on and get worse (and they would do a five hour drive and turn up on the doorstep) but that if he did ring back FIL would rant at him and then MIL would cry a lot.