Perhaps she thinks this is a good time to talk. She probably doesn't want to bother him at work or during the work week at night. Maybe she thinks Saturday and Sunday are too busy for you. Everyone today loves to tell everyone else how busy they are. It always feels like it is never a good time to call anyone. Older generations are use to talking on the phone. When do you think it would be a good time for her to call. Maybe your DH could just tell her a time that is better to call.

This isn't a MIL problem, it's a DH problem.

Another "huh"? here. I have MIL issues too, but I don't see the problem here.

My MIL is an alcoholic.

Huh?
U don't have a cordless phone or cell phone?
He can't wash dishes or give kids a bath while in the phone?
I do that s*** everyday.

You also have a husband problem.

If it is an inconvenient time, look at the ringer and ask your husband to ask his mother to call back at XYZ time. Don't pout. And she's not a mind reader.

Oh, and start making plans for weekly Friday nights out so that night is no longer convenient for phone calls.

Try to wean her to Sunday afternoon OP.

That is what my husband and his mom do.

WAY better than Friday night.

I'll bite. I'm projecting...but "once again" meaning..often?
A friday night so maybe OP was expecting some quality time after dinner? Maybe she would like some help getting the kids cleaned up and ready for bed?
An hour is not so much if once every few weeks..but otherwise..it does seem like a lot.

Huh?

Why shouldn't she talk to her son?

I don't understand. Your mil wants to talk to her son. She calls after dinner. What exactly are you upset about OP?

What's the problem here?

I don't understand the issue.

Once again, my MIL calls just as we finish dinner on a Friday night.

Then she and her beloved son talk for an hour....

Not the worse thing ever in the whole world, but annoying.