What you are describing is a hosted UC service. Cisco has some documentation around their own offering that may give you some insight. Start here:
http://www.cisco.com/en/US/partner/prod/collateral/voicesw/ps6788/vcallcon/ps8362/product_data_sheet0900aecd80670040_ps556_Products_Data_Sheet.html
As a thought exercise, we can explore this some. You would clearly need to provide call processing at your fictitious data center. You may also need to provide basic applications such as voicemail. Multitenant features in CUCM and Unity Connection can be leveraged to create a robust and secure solution.
For PSTN connectivity, you will need to hand off to a PSTN carrier in some way. It could be a SIP trunk, PRI, etc. Which means you have to manage the costs related to managing and provisioning. You also need to reliably provide services such as 911. This will be a huge consideration. You can't just hand off a 911 call to the PSTN and assume that everything works. You have to route emergency calls to the correct PSAP. By correct I mean route calls to the PSAP that services the area where your fictitious customer sites reside. There are lots of regulatory considerations around this topic. You would need to do more research. Perhaps google search "
E911 hosted IP PBX".
As far as network connectivity between the hosting facility and the customer prem. This is basic networking 101. You would likely partner with some MPLS provider that has a POP in your fictitious data center and then have some sort of mutual service offering. Maybe you could establish agreements with multiple MPLS providers?
You could also think about using customer prem equipment (CPE). Where you drop off a multi-service router of some type that can provide local PSTN connectivity (working around your 911 issue, perhaps) and other security-centric services. Value add for your overall solution perhaps.
Those are my random thoughts on the topic.
HTH.
Regards,
Bill
