Work cell phone question for the TB IT managers

We are trying to streamline our companies cell/myFi usage. We are about half and half company-provided cells with myFis and personal cells with a stipend and a company provided myFi. .One option we are exploring is to get rid of any myFi devices and move to running hotspots on the phones for everyone. What we are trying to determine is how to handle the personal cells in that situation.

Regardless of what the employee's personal cell bill is, we give everyone the same amount which works out to roughly what it would cost if the cell was company-provided. If we go the hot-spot route for the personal cells, we are concerned about the employees not having enough data on their plan and going over and getting charged and then complaining that we forced them to go over their data and wanting to up their stipend. How would you ensure the employees have an appropriate plan to serve what we need them to use it for? Does anyone have experience with an agreement for personal cells?