I'm with
@Brian706, I think a landline is a great solution. It would completely eliminate any of the issues a forgetful person could cause. Your mother-in-law would no longer need to remember to charge her phone. It couldn't die because of a dead battery. It couldn't lose its connection if she's in a 'dead' spot in the house. She couldn't shut it off. (She could turn off ringers, though.)
Due to my location [near mountains] my cell service is spotty, so I've kept my landline and use it as my primary phone. You could set your MIL up with something similar to mine: a hardwired base unit plus as many cordless phones as desired. I have the base unit in the den, then cordless stations all around the house. They don't need access to phone jacks; they're "registered" with the base unit when they're first set up, then they're free to use wherever, even outside. The bases are also chargers, so when the phones are in their bases, they're recharging.
One other idea is to set her up with an emergency alert system. There are many available, with different features and requirements, so I won't bother mentioning any. You can research those if you're interested. If she already has an alarm system, like ADT, adding an emergency system is easily done. This wouldn't let you directly contact her, like when she's shut off her phone, but it would give her the ability to seek help if she needed it--and her phone was dead, or in another room, etc.
Ultimately, as I'm sure you're aware, she's going to need full-time care and assistance. But a landline would address the immediate issues.
