need dialplan for randomising two carriers

I don't have documentation on randomizing outbound calls between two carriers. The dialplan patterns covered here are all deterministic: matching on dial prefix and number pattern, least-cost lookup by NPANXX, or primary/backup failover triggered by specific SIP responses.

Here's what's actually documented, in case one of these fits your need:

Deterministic pattern selection

Carrier choice normally comes from which dialplan pattern matches the dialed number, not randomness: VICIdial hands the number to Asterisk, Asterisk compares it against a list of patterns, and the first matching pattern decides which carrier carries the call. When two patterns could match, Asterisk uses most-specific-wins matching: a more exact pattern beats a looser one. If you have a broad catch-all and a tighter pattern for a specific number range, the tighter one takes the call.

Least-cost routing (rate-based, not random)

If the goal is actually to split traffic between two carriers by economics rather than by chance, agi-LCR-Route.agi does something related: before a call is placed, it checks a rate table and picks the lowest-cost carrier string for the number being dialed. VICIdial implements this through agi-LCR-Route.agi, an AGI script that runs inside the Asterisk dialplan and sets a variable the Dial() command reads. This always picks the cheaper of the two carriers for a given prefix, it does not alternate or randomize.

Failover (only on failure, not random)

A backup carrier setup only shifts traffic to the second carrier when the first one fails: A backup carrier is a second carrier plus routing that automatically retries on the backup when your primary trunk cannot complete a call. When the primary returns a sip-503 or congestion, the dialplan falls through to the backup instead of just dropping the lead.

Since a true random split isn't covered in what I have, I'd rather not guess at dialplan syntax for it. Drop a note at /contact and describe the split ratio you want, someone can confirm whether it's supported and how to wire it.