Choufleur we may well be related and sharing the same MiL. Mine has control and boundary issues and is a bit extream in her behaviour.
She had a habit of calling us at the worst time possible, for example ringing at 6:55pm and then saying something like "I know you said you were going out at 7pm so I just thought I would catch you.." but taking offence that she was then rushed off the phone because we had to leave the house. It wouldn't even be anything important that she was ringing for, she'd actually say "I don't know why I rang, we haven't been anywhere or done anything to tell you about..." It was more the fact that she knew it was her last chance to call and so she was taking it.
If she knew we wanted to watch a certain programme at 8pm she'd be on the phone at 7:59pm to remind us it was about to start and then she'd want to talk to both of us for an hour and we'd miss it.
If we didn't answer the phone she would phone without fail every fifteen minutes until we answered. If she still didn't reach us then FiL would ring and leave an aggressive message to say she was crying and it was our fault.
She once rang my parents 17 times in the space of two hours to ask them if they knew I was missing (they were at work when she started to call) and I wasn't missing, I'd gone out to my book group and missed the other 17 calls she had made to us at home).
The calls were almost always when she knew for certain that we had other plans or would be busy at just that particular moment.
YANBU. Here's how I dealt with the problem.
I recorded a message on the answerphone that said "Mr and Mrs Takethis...are not taking calls right now, please leave a short message with your name and number and if we want to call you back we will get back to you tomorrow. Please don't keep calling in the meantime as we are unable to come to the phone right now."
I also stuck a big sign on the door saying "do not knock, telephone or otherwise disturb, new mum and baby are sleeping."
That did help to keep her calls and visits at the wrong time down a lot.
Perhaps you could do the same, leave a message on the answerphone saying not to call after 6:30pm because it's your busiest time with the baby and that anyone who does call at that point will have to be called back the following day when you have more time.