I would suggest it is just a cost of doing business, but it is maybe a case of how senior your execs are. Do you really want them messing round with prefixes, etc.

If you want to reduce costs just tell them how much they're spending and that it will have a direct impact on next year's pay/bonus, etc.

And if they're really spending $2,500 a month then I'd be talking to the phone company - if you're putting that amount of business through them I'm sure they'd be happy to discuss their pricing (or even, if you're that way inclined, to place a cap on their roaming data allowance). Just tell them you're moving to using roaming sims and see what their response is.

Another solution is to disable roaming altogether and tell everyone that if they are travelling overseas they must get a sim from you before they travel. It would probably be better value to get a bunch of contracts set up in each country which can be freely shared. $50 a month is still going to be way cheaper tham using roaming.