Thanks for posting the question. I realize that you are asking a question of Pega customers (particularly of customers with outsourced call center models with several clients) - and hopefully you'll still get that kind of response, but I thought I'd chime in here, as the use case is an intriguing one.
Essentially there shouldn't be anything stopping you doing something like that with PegaCall. If that agent is receiving a call from any one of those 10 tenants/customers - then you'd need to make sure that the CTI/ACD solution that PegaCall integrates into is handling the separate lines etc for each of those 'tenant' brands. The CSR logs in once into PegaCall, and their availability is for any one of those brands, they wouldn't be able to switch availability per brand, they are available, on a call or not available essentially.
Give those agents skills per brand/tenant. It’s should also be possible to map the DNIS from a number associated with a brand to a flow for each client, allowing you to treat them differently and set up dialogs/scripting for interactions/cases accordingly. Hopefully those tenants/brands are similar enough that the cases available to the CSR make sense for all of them!