IP Multimedia Subsystem is an IP based signaling system for setting up and tearing down multimedia sessions. SIP based signaling is used to setup these sessions.
We have covered call flows for an IMS to IMS and PSTN to IMS calls. We now look at the call flow for a IMS to PSTN subscriber call.
The call is routed via the BGCF (Border Gateway Control Function) to the MGCF (Media Gateway Control Function). The MGCF uses one context with two terminations in IM-MGW (Media Gateway). The termination RTP1 is used towards IMS Core network subsystem entity and the bearer termination TDM1 is used for bearer towards PSTN CS network element.
The call flow is complex and it is analyzed with multiple diagrams. Some of the diagrams are presented here:

A high level view that abstracts out individual component details and just presents the flow between the UE, IMS Core and the PSTN.

Here we examine the call flow from the UE point of view. The call flow is represented as a collaboration diagram.
Link: IMS subscriber to PSTN subscriber call flow
All documents:
- IMS to PSTN Call Flow
- IMS to PSTN Call Flow Poster (11×17)
- IMS to PSTN Call Overview
- IMS to PSTN Call Context Diagram
- High Level Flow (Calling UE-IMS-PSTN)
- High Level Flow (With CSCF and H.248 Details)
- IM-MGW Call Flow
- IM-MGW Context Diagram
- MGCF Call Flow
- MGCF Context Diagram
- BGCF Call Flow
- BGCF Collaboration Diagram
- Originating UE Call Flow
- Originating UE Collaboration Diagram