Trunk groups#
An inbound Trunk Group combines multiple SIP trunks and PSTN trunks into one routing destination. Use a Trunk Group when calls to a DID number must fail over to a backup destination, be distributed among equivalent destinations, or both.
A Trunk Group can contain SIP trunks, PSTN trunks, or both. Each trunk can belong to only one Trunk Group. The group defines a shared Capacity Limit, and each member trunk defines its own Priority and Weight within the group.
How trunk groups work#
When a caller dials a DID number assigned to a Trunk Group, DIDWW evaluates the member trunks by Priority. Trunks with lower priority values are preferred. When multiple available trunks have the same priority, Weight controls their relative likelihood of selection.
If an attempt meets the configured re-routing conditions, DIDWW tries another eligible member trunk. Each routing attempt generates a separate Call Detail Record.
%%{init: {
"theme": "base",
"themeVariables": {
"fontSize": "14px",
"primaryColor": "#e0f2fe",
"primaryBorderColor": "#38bdf8",
"primaryTextColor": "#1f2d3d",
"secondaryColor": "#ccfbf1",
"secondaryBorderColor": "#2dd4bf",
"secondaryTextColor": "#1f2d3d",
"lineColor": "#0066cc",
"textColor": "#1f2d3d",
"nodeTextColor": "#1f2d3d",
"edgeLabelBackground": "#ffffff"
}
}}%%
flowchart LR
caller["Caller"] --> |dials DID| did["DID number"]
did --> |assigned trunk| group["Trunk Group"]
group --> |Priority and Weight| member["SIP or PSTN trunk"]
member --> destination["Destination"]
member -.-> |re-routing| group
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,group,member process
linkStyle default stroke:#0066cc,stroke-width:1.5px
Trunk groups and DID numbers#
A DID number receives the inbound call, while the Trunk Group defines which trunks can deliver it. To receive calls, assign the Trunk Group to a DID number. Multiple DID numbers can use the same Trunk Group.
The DID number must have sufficient Capacity for concurrent inbound calls. The Trunk Group’s Capacity Limit can further restrict simultaneous calls, but it cannot increase DID Capacity.
Priority and Weight#
Use Priority to create routing tiers and Weight to distribute calls within a tier. Both are set on each member trunk. See RFC 2782 for the selection model.
Objective |
Configuration |
Behavior |
|---|---|---|
Primary and backup routing |
Assign a lower Priority to the primary trunk and a higher Priority to the backup trunk. |
DIDWW prefers the primary trunk. DIDWW tries the backup tier only after no eligible trunk in the preferred tier can complete the call under the configured re-routing conditions. |
Load balancing |
Assign the same Priority to multiple trunks and configure their Weight values. |
DIDWW distributes calls among the trunks. A higher Weight increases a trunk’s relative likelihood of selection. |
Load balancing with failover |
Create multiple priority tiers, each with equal-priority trunks. |
DIDWW balances calls within the preferred tier and continues to a later tier when re-routing applies. |
Capacity and re-routing#
The Trunk Group Capacity Limit restricts the total number of simultaneous calls delivered through the group. Calls that exceed the group limit are rejected. Each member trunk can also have its own Capacity Limit. When a member trunk reaches its limit, DIDWW can attempt another eligible member.
For SIP members, Re-routing Disconnect Codes determine which failed attempts continue to another trunk. A member trunk’s Ringing Timeout can also end an attempt when the destination does not answer in time. For routing-attempt and Call Detail Record behavior, see Service failover and load balancing.
Get started#
Create a group from your SIP and PSTN trunks, assign it to a DID number, and test call delivery.
Combine existing SIP and PSTN trunks into a group and configure its capacity.
Assign the group to one or more DID numbers to receive incoming calls.
Call an assigned DID number and review the delivery attempts in Inbound Call Logs.
Manage and configure routing#
Update the group or configure how its member trunks are selected.
View or edit group settings and membership, or delete groups that are no longer needed.
Set Priority, Weight, and SIP re-routing conditions on individual member trunks.
Delivery behavior and references#
Look up routing behavior and configuration fields.
Understand how DIDWW selects member trunks and when it tries another eligible destination.
Look up group capacity, membership, Priority, Weight, and re-routing settings.