I suspect early dementia - when someone starts to behave inappropriately and intrusively it often means that the parts of their brain that restrain their behaviours and tell them what is and what is not reasonably acceptable have started to wear out.
I am widowed and live alone - and am nowhere near 85! - but know how hard it can be not to contact family when you are feeling bleak. The things that stop me are an awareness of their busy lives and being able to remember how I was at their age. I still have the tools in my brain to stop me over-stepping the mark and becoming a bloody nuisance - and I lead a busy life.
If MIL is doing this even though she has her OH there, then I would suspect that something is beginning to go awry in her brain. As to what you do........ you could stop picking up every time, and then explain you were busy when you next speak to her. Or you could have set times for meeting her each week. Is FIL with it? Does he realise what she is doing and know it is a bit out of order? Can he intervene?
Could you simply say, very kindly, I am sorry but we are often quite busy and cannot pick up, but we will see you as arranged each week (or whatever you arrange).
Your OH sees no problem, but is he on the receiving end of the calls? If it is to a home phone then maybe get him to answer? Or you could divert the calls to you to him.
Or could you fix up WhatsApp? - or is she not able to cope with this? This is how I keep in touch with my AC - we make very brief contact several times a day.
It sounds as though things are not going right for this poor lady. I hope you can find a reasonable solution that takes into account everyone's needs.