This is why, if work wants me to have cellular connectivity, they can buy me an appropriate phone and plan to go with it. It's not my responsibility to have a high reliability plan on my personal phone if I don't care (and I don't - I currently carry a flip phone).

I've played the "combined personal and work phone" game in the past, rather extensively, and have come to the conclusion that it's a sucker's game, because it means you always have access to work - and will therefore spend a lot more time thinking about it than you would with separate devices.