Interesting observation sadwidow, but I think you should be on the downtown thread!
From my reading of the OP's post, she has caller ID and an answerphone, but her mum rings and rings anyway and doesn't leave a message.
Her mum rings the home phone and uses a withdrawn number as do some clients, so op has to pick up.
Op also uses her mobile for most of her business calls.
OP's mum rings and rings whenever a thought comes into her head, so she rings at all hours,
The issue is a lack of respect for women who work from home as op has a brother who she doesn't phone all day.
Now, how to help op who is already doing everything suggested.....
Op maybe phone your mum in the evenings every two days, and say very clearly that you've got a lot on and clients are using the phone all day.
During the day.. Tell your mum that you're expecting a client to call, so can't talk, but will phone her later.
Remind your mum that you're on a deadline and will phone her later at 7 or some nominated and mutually agreed time. And stick to it.
Finally and what gets my mum off my back when I'm working at home is that I ring my mobile when on the phone, (I have a mobile phone with a 'ring yourself' button) as soon as my mum calls, I activate a call yourself, and I say "oh mum, a client is ringing, and I have to go, money's money!"
Or I press a beeping button on the landline handset and say there's a client trying to get through, so I have to go.
Persistent goodnatured training is essential. Be cheery but firm.
"I can't talk to you now, I'm expecting a call, I'll ring your at 7 would that suit?" Every every time.
At least she isn't turning up for coffee at all hours, as mine did until I just bustled her out one day, and told her to go to Starbucks!