You can accept that employees will use corporate-owned devices for personal use and train them how to use data services efficiently. Your employees are smart, and if you explain to them that if they pay for local Wi-Fi at a hotel (or use free Wi-Fi) versus using 3G/4G for data streaming, they are going to save the company money and not go over the data plan. If employees are traveling internationally, get them on an international plan. You don't want to pay international roaming data charges under any circumstances.
You could ask your provider for discounts--which you should be doing anyway--or about pooled accounts. Pooling is similar to family plans. You get x amount of data per month shared across all of your users. Some users won't use much data, while some will be heavy users. It should even out in the end. Just make sure that you can reconcile the bill at the end of the month. If you are going over on data, identify the data hogs and address the issue with them individually.
Remember, some of these data hogs are sales people who are bringing in hundreds of thousands or millions of dollars in business while on the road. Going over on a data plan because they stream the latest video is probably not worth sweating. Consider it a perk. So-called data hogs may have legitimate reasons why they go over, and you might have to move them into a different tier.
If you are supporting a bring-your-own-device model where employees are reimbursed for their data plans, you can set a fixed amount that the company will cover for a data plan based on estimated work-related use. This plan works very well when your company has an employee discount plan with a carrier. Typically, data plans start at 2-Gbyte chunks and go up from there.
There are likely many more strategies to manage telecom costs. The key is to balance cost with convenience, especially if you are talking about traveling employees, and to come up with a plan that meets the needs of groups of users.