Using a Dial Prefix to Pick a Carrier in VICIdial

A dial prefix is one of the most useful tricks in VICIdial carrier routing, and one of the most misunderstood. It's just a digit (or two) that VICIdial sticks on the front of every number a campaign dials. That extra digit never reaches the phone network; its only job is to tell Asterisk which carrier should carry the call. Set it up right and you can point different campaigns at different carriers without touching a single lead.

This post explains what a dial prefix is, how it steers a call to a specific trunk, and how to avoid the off-by-one mistakes that send calls nowhere.

What the prefix actually is

A Dial prefix is set on the campaign. When an agent dials, VICIdial prepends it to the number before handing the call to Asterisk. So a campaign with prefix 9 turns 18005551212 into 918005551212. Asterisk then matches that against Dialplan patterns, and the pattern that starts with 91 belongs to one specific Carrier. Change the prefix to 8 and a different pattern, pointing at a different trunk, picks up the call instead.

The prefix is stripped back off before the number leaves your server, using ${EXTEN:n} in the Dial line. The carrier only ever sees the real number; the routing digit is purely internal.

One prefix per carrier

The pattern is simple: give each carrier a distinct leading prefix, then set each campaign's dial prefix to the carrier you want it to use. Carrier A owns patterns starting with 9, Carrier B owns patterns starting with 8, and so on. Now campaign selection of a carrier is just a one-field change on the campaign, no dialplan editing required after the initial setup.

How the prefix steers the call

flowchart TD
  A[Agent dials number] --> B[Campaign adds dial prefix]
  B --> C{Prefix value?}
  C -->|9| D[Match pattern 91...]
  C -->|8| E[Match pattern 81...]
  D --> F[Strip prefix and dial Carrier A]
  E --> G[Strip prefix and dial Carrier B]
  F --> H[Carrier A to PSTN]
  G --> I[Carrier B to PSTN]

The strip count has to match

This is where most prefix setups break. If you add one digit (9) you must strip one digit (${EXTEN:1}). If you add two (91) you must strip two (${EXTEN:2}). Get the count wrong and the carrier receives a number that's one digit short or carries a stray 9 on the front, and it rejects the call. Whenever you change a prefix, recheck the strip count in that carrier's Dial line.

• Campaign prefix digits must equal the leading digits of the carrier's pattern.
• The strip count in ${EXTEN:n} must equal the number of prefix digits.
• Each carrier needs its own non-overlapping prefix and pattern.

How this fits the bigger picture

Prefix-based selection is the manual cousin of automatic routing. If you want the system to choose the cheapest path instead of you choosing per campaign, that's least-cost routing. If you want a campaign to keep dialing when its carrier dies, layer in multi-carrier failover. All three sit on top of the same pattern matching covered in the carrier integration guide.

A dial prefix turns carrier selection into a single campaign setting. Assign each carrier its own prefix and pattern, keep the strip counts honest, and you can move whole campaigns between providers in seconds. VICIfast provisions a dedicated dialer with editable carrier patterns in under 40 seconds. See our pricing to put your carriers to work.