+---------+
                          /           \
                      +---             +---+
+----------+  +--+   /                      \   +--+  +----------+
|          |  |  |  |        Gateways        |  |  |  |          |
|   Alice  |<-|GW|->|         and/or         |<-|GW|->|    Bob   |
| (caller) |  |  |  |          PSTN          |  |  |  | (callee) |
+----------+  +--+   \                      /   +--+  +----------+
                      +---             +---+
                          \           /
                           +---------+