Hello Timothy
RSVP is used in Cisco Unified Communications architecture in a feature called Call Admission Control (CAC) makes a bandwidth reservation using RSVP along the full length of the path between the two communicating endpoints. Look at the following diagram that comes from Cisco documentation (the link to this document is shown further below).
There is a SIP session that is initiated between two user agents (UAs). These can be phones, gateways, conference bridges etc, anything that will terminate a SIP session. The SIP session begins with INVITE, SESSION PROGRESS, and PRACK OK messages between the two devices. Once that is done, the SIP UA1 initiates an RSVP reservation to UA2. This process involves all RSVP enabled routers between the two devices. Once that has successfully been established, the SIP session continues with an exchange of UPDATE messages, and then the phone rings.
So RSVP takes place chronologically between the initialization of the SIP session and the beginning of the actual telephony communication starting with the ringing of the phone.
This whole mechanisms is a subset of a broader concept of CAC which is fundamental for Cisco’s implementation of IP telephony. You can find out more information about this specific portion of CAC at the following link:
Also, a very elegant explanation of RSVP and how it works can be found in the same document at this location:
I hope this has been helpful!
Laz