Can you explain to your Mum that there isn't any necessity to answer the 'phone, no need to jump up from the stairlift for example. When I was ill I would pull the plug from the wall so that I wasn't disturbed, however, your Mum might not remember to replug. Does your Mum have an answer phone so that she can allow any call to go there. She can then ring you when it's you?
There are boxes that one can buy that filter these calls apparently.