I'm going to assume the ATLAS has the PRIs configured under the DIAL PLAN. In the DIAL PLAN, all calls are routed based on the destination number (DNIS - Dialed Number Identification Service). This is done through the IN#ACCEPT (under the USER TERM configuration) and the OUT#ACCEPT (under the NETWORK TERM configuration).
If you want a call to "roll-over" from one PRI to another, then the IN#ACCEPT (or OUT#ACCEPT) for both PRIs should have the same ACCEPT NUMBER. (They can have additional ACCEPT NUMBERS, but the number that you want to roll-over must be in both.) If you are using "wild-cards" (like "$" or "XXXX") then both PRIs must have the same "wild-card" match to allow the call to roll-over.
As far as a guide goes, this one may help: https://supportforums.adtran.com/docs/DOC-1392
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