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 or the phone.systems™ PBX.
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"]
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class caller,destination endpoint
class did,trunk,termination process
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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. Multiple DID numbers can use the same PSTN trunk and forwarding destination.
The DID number must have sufficient Capacity for concurrent inbound calls. The trunk’s Capacity Limit 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 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 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.
Get started#
Create a trunk, assign it to your DID number, and test call forwarding.
Set the forwarding destination, check its per-minute rate, and create an inbound PSTN trunk.
Assign the PSTN trunk or its trunk group to one or more DID numbers.
Call an assigned DID number and review the forwarding attempt in Inbound Call Logs.
Manage trunks and call handling#
Review existing trunks and adjust their call limits and caller filtering.
View destination rates, edit trunk settings, or delete trunks that are no longer required.
Look up the limits for simultaneous calls and how long DIDWW waits for the destination to answer.
Assign a number list to your PSTN trunk to allow or reject calls based on caller ID.
Failover and load balancing#
Use these guides when your PSTN trunk will share call delivery with other trunks.
Combine existing SIP and PSTN trunks into one routing destination.
Set member trunk Priority and Weight to control attempt order and call distribution.
Understand member selection and when a failed attempt can continue to another destination.
Technical references#
Find field definitions, defaults, supported values, and limits for PSTN trunk settings.