My Mum can't hear her phone ring upstairs what can I do?

The only way I can fix this very situation with my mother is to call the Memory Care AL where she lives and ask the staff to go into her room to hang the phone up for her. Otherwise, it'll stay off the cradle, so to speak, for hours on end. Things reach a point with Alz/dementia where a phone is no longer a viable option b/c it's just too confusing a thing for them to operate. Hopefully your mother isn't living alone?

My DH is very hard of hearing. Our main phone is connected to phone lines and mounted on the wall. I have the ringer as loud as it will go, my DH can hear it. With that phone came a cordless one with a recharging stand. The cordless phone does not have to be on the stand to ring. The battery lasts for a period of time before needing to be put on the stand to recharge. If the cordless is near my husband he has no problem hearing the ring. I put the voice volume up loud for him and the phones also have a "boost" button for a little extra volume. These were purchased at radioshack some time ago. There are phones out there for the hearing impaired. Maybe see if there is one that when it rings a light blinks.

I purchased a ringer from Amazon that you can attach. It's so loud that imho it can wake the dead. That may work.

There comes a point where certain useful tools can no longer be used by our LOs. Have you considered going back to an old-school phone that doesn't require batteries? Even then she will at some point either not hear it or forget what the ringing sound means. The other option is to install a 2-way intercom controlled at your end, so that you can speak to her and then when she speaks you can hear her. Not sure all this effort will last for long as she continues to decline, but you are the best judge of that. Hoping you find something simple that works!