Cheap dial-up options

Our company has 5 locations that use 5 T-1’s for data. Each one of these locations has about 5 people with laptops that are required to check email after 5pm and on weekends using a dial-up account. I don’t want to pay much for these 25 people to access the Internet on off hours, but I want unlimited access in case of the incidental hefty overage, they’re inevitible. What should I do? I would think I could get a simple business access number (for dialing in only) at $5 bucks/per person or less per/mo but I’m not finding one, especially w/nationwide access numbers. Help!