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