So the Question Is

What will my network have to do to support this (IMS) functionality?

Some differences to consider include:
Voice Architecture
Packet Switched v. Circuit Switched architectures
In-Band (IP) v. OOB (SS7) signaling
QoS and Billing Architectures
Provider mediated sessions (QoS, Policy) v. end-to-end principle
We'll talk more about the implications of these in a minute
Walled Garden v. "Internet"  business models
...