# Learn about using Amazon Chime SDK PSTN audio service call legs
<a name="call-architecture"></a>

The PSTN audio service can operate on one or more call legs. For example, you have a single call leg when you record or deliver a voice mail, and you have multiple call legs when you join an Amazon Chime SDK meeting.

The following diagram shows the flow of a single-leg call.

![\[Diagram of the architecture of a single call leg.\]](http://docs.aws.amazon.com/chime-sdk/latest/dg/images/single-leg-architecture.png)


The following diagram shows the architecture of a multi-leg call.

![\[Diagram of the architecture of a multi-leg call.\]](http://docs.aws.amazon.com/chime-sdk/latest/dg/images/multi-leg-architecture.png)


The following diagram shows the flow of a multi-leg bridged call.

![\[Diagram of the architecture of a multi-leg bridged call.\]](http://docs.aws.amazon.com/chime-sdk/latest/dg/images/Multi-Leg-Architecture-w-Bridge.png)