The best prepaid SIM card for roaming

[bylo]

If reliability is important, Mobal is your best option. As for cost, they have a referral program under which you can save up to 50% on the cost of calls. If you plan to buy a "large quantity" of SIMs then perhaps you should contact them about making your company a special deal. They also have a reseller program because I've seen their products sold by others. Again contact Mobal for more information.

As for the others -- and you should include United Mobile in that list -- search this forum for threads about problems with their service in certain countries. If that's acceptable to you for wide deployment within your company then by all means consider those options too.