Callan served in Starfleet prior to joining the Maquis. His records showed he had seen combat.
In 2372, he travelled by freighter to Tavvis II to met Brendan Patol and join the Maquis. He went to the meeting place, a bar, and while waiting was accosted by three Human males. In the ensuing brawl he disabled two with a personal stunner and broke the wrist of the third, gaining that man's phaser.
Patol appeared and revealed that Callan had just experienced a holographic test of his skills. Patol gave Callan a chance to reconsider joining, which Callan turned down. This resulted in Callan joining Patol's Maquis cell. ("Initiation")