# PSTN trunks

An inbound **PSTN trunk** forwards calls received on your DIDWW DID numbers
to a telephone number on the Public Switched Telephone Network (PSTN), such
as a landline or mobile number. Use this trunk type when incoming calls
should reach an external phone number rather than a [SIP endpoint](../sip-trunks/index.html) or the [phone.systems(TM)
PBX](../phone-systems-trunks/index.html).

The **PSTN Phone Number** configured on the trunk is the forwarding
destination, not the DID number that callers dial. The trunk also provides
settings for limiting simultaneous calls, controlling the ringing timeout,
filtering calls by caller ID, and joining a Trunk Group for failover or load
balancing.

## How PSTN trunks work

When a caller dials a DID number, the public telephone network routes the
call to DIDWW. DIDWW uses the trunk assigned to that number to determine
where to deliver the call. For a PSTN trunk, DIDWW re-originates the call
to the configured **PSTN Phone Number** through DIDWW termination.

```
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}}%%

flowchart LR

    caller["Caller"] --> |dials DID| did["DID number"]
    did --> |assigned trunk| trunk["PSTN trunk"]
    trunk --> |re-originates call| termination["DIDWW termination"]
    termination --> destination["Destination phone number"]

    classDef endpoint fill:#e0f2fe,stroke:#0066cc,color:#1f2d3d,stroke-width:1.5px
    classDef process fill:#ccfbf1,stroke:#2dd4bf,color:#1f2d3d,stroke-width:1.5px

    class caller,destination endpoint
    class did,trunk,termination process

    linkStyle default stroke:#0066cc,stroke-width:1.5px
```

## PSTN trunks and DID numbers

A DID number receives the inbound call, while the PSTN trunk defines the
forwarding destination. To receive calls, [assign the PSTN trunk or a
Trunk Group containing it to a DID
number](../../../phone-numbers/my-numbers/how-to-guides/assign-voice-trunk.html#user-panel-assign-trunk).
Multiple DID numbers can use the same PSTN trunk and forwarding destination.

The DID number must have sufficient [Capacity](../../../phone-numbers/capacity/how-capacity-works.html) for concurrent
inbound calls.
The trunk's [Capacity Limit](pstn-trunk-settings-reference.html#inbound-pstn-trunks-settings-general) can
further restrict simultaneous calls, but it cannot increase DID Capacity.

## Rates and destination availability

Note

Calls forwarded through a PSTN trunk are charged per minute. The rate
and availability depend on the destination **PSTN Phone Number**.

When creating or editing a trunk, **Check Rate** confirms destination
availability and displays the current rate. Check the rate again when
changing the destination. See [DIDWW
pricelists](../../../phone-numbers/buy-numbers/download-pricelists.html#services-coverage-pricing-access-pricelists) for
current pricing.

## Failover and redundancy

A PSTN trunk forwards calls to a single destination. To provide failover or
load balancing, add the trunk to a [Trunk Group](../trunk-groups/index.html)
with other SIP or PSTN trunks and assign the group to the DID number. A trunk
can belong to only one Trunk Group.

For example, use a SIP trunk as the primary member and a PSTN trunk that
forwards calls to a mobile number as a backup. For how trunks in a group
are selected, see [Service failover and load
balancing](../sip-trunks/call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-failover-load-balancing).

---

## Get started

Create a trunk, assign it to your DID number, and test call forwarding.

**Create a PSTN trunk**

Set the forwarding destination, check its per-minute rate, and create
an inbound PSTN trunk.

[Create a PSTN trunk](how-to-guides/create-pstn-trunk.html)

**Assign a voice trunk**

Assign the PSTN trunk or its trunk group to one or more DID numbers.

[Assign a voice trunk](../../../phone-numbers/my-numbers/how-to-guides/assign-voice-trunk.html#user-panel-assign-trunk)

**Test and view call logs**

Call an assigned DID number and review the forwarding attempt in
Inbound Call Logs.

[Inbound Call Logs](../../../logs-analytics/call-logs/inbound-logs.html#inbound-cdr-logs)

## Manage trunks and call handling

Review existing trunks and adjust their call limits and caller filtering.

**Manage PSTN trunks**

View destination rates, edit trunk settings, or delete trunks that
are no longer required.

[Manage PSTN trunks](how-to-guides/manage-pstn-trunks.html)

**Capacity and ringing timeout**

Look up the limits for simultaneous calls and how long DIDWW waits
for the destination to answer.

[General](pstn-trunk-settings-reference.html#inbound-pstn-trunks-settings-general)

**Configure caller filtering**

Assign a number list to your PSTN trunk to allow or reject calls
based on caller ID.

[Assign a Number List to a PSTN Trunk](../number-lists/how-to-guides/configure-number-lists.html#inbound-number-lists-assign-pstn)

## Failover and load balancing

Use these guides when your PSTN trunk will share call delivery with other trunks.

**Create a trunk group**

Combine existing SIP and PSTN trunks into one routing destination.

[Create a Trunk Group](../trunk-groups/how-to-guides/create-trunk-group.html)

**Configure member routing**

Set member trunk Priority and Weight to control attempt order and
call distribution.

[Configure member trunk routing](../trunk-groups/how-to-guides/manage-trunk-groups.html#inbound-trunk-groups-manage-member-routing)

**Trunk group failover and load balancing**

Understand member selection and when a failed attempt can continue
to another destination.

[Trunk Group failover and load balancing](../sip-trunks/call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-trunk-group-failover-load-balancing)

## Technical references

**PSTN trunk settings reference**

Find field definitions, defaults, supported values, and limits for
PSTN trunk settings.

[PSTN trunk settings reference](pstn-trunk-settings-reference.html)

## Related resources

Explore caller-filtering options and investigate delivery problems.

**Number lists**

Create and manage caller-filtering lists and explore example rules.

[Number lists](../number-lists/index.html)

**Inbound calls troubleshooting**

Identify why an inbound call failed and find guidance for PSTN
delivery problems.

[Troubleshooting](../troubleshooting.html)

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