What would be the best solution for a business that has office space divided up into sub-offices, which it leases to other companies, and would like to offer services for the leasers, such as Internet access (T1) and voice mail? There will be an administrator to service the leasers.
You would need to become an ISP yourself. I'm not sure this is a feasible (i.e. cost-effective) solution for you unless you have many clients who are willing to pay a premium for the service. You'll need to establish separate subnets for each company, strong security divisions between each company's traffic, firewall protection and proxy services. If you want to offer wireless connectivity, you've got even more issues, including cross-site trafficking, confidentiality and access by external entities.
Or you could just find several leading local or national ISPs in your area and ask them about providing this service to you and your clients.