ZOB ASSOCIATE CTLR DESCRIBED OPERROR AT APPROX FL260 WHEN CTLR ISSUED CTL CLRNCS TO ACFT WITH SIMILAR CALL SIGNS RESULTING IN A CONFLICT.

Narrative

I WAS ON THE RADAR ASSOCIATE POS DURING A MODERATE TO HVY SESSION. THE RADAR CTLR WAS WAITING TO DSND A WBOUND REGIONAL JET AS A TWIN CESSNA WAS XING SBOUND. I ANSWERED A HDOF LINE FOR COORD. WHILE I WAS OFF FREQ; THE RADAR CTLR GAVE TFC TO THE REGIONAL JET. THE RADAR CTLR ONLY USED THE LAST 3 DIGITS OF EACH CALL SIGN (BOTH CALL SIGNS ENDED WITH THE SAME DIGITS). THE RADAR CTLR TOLD THE WBOUND REGIONAL JET TO MAINTAIN FL270 AND THERE WAS TFC SBOUND -- A TWIN CESSNA AT FL260. WHEN I CAME BACK ON FREQ; 2 ACFT WERE TALKING. THE RADAR CTLR IMMEDIATELY TOLD THE TWIN CESSNA TO MAINTAIN FL260 AND HIS TFC WAS WBOUND AT FL270. THE TWIN CESSNA SAID THAT HE WAS GOING BACK DOWN. THE RADAR CTLR TOLD HIM TO RPT LEVEL. THE TWIN CESSNA DID; BUT SEPARATION WAS LOST DURING HIS SHORT CLB. THE RADAR CTLR SHOULD HAVE DONE 2 THINGS TO AVOID THE CONFUSION: 1) ADVISE BOTH ACFT THAT SIMILAR CALL SIGNS WERE ON FREQ; 2) USED FULL CALL SIGNS ON EITHER OF THOSE 2 ACFT ON ANY XMISSION TO THEM. I COULD HAVE ADVISED THE RADAR CTLR TO ADVISE THE 2 ACFT OF SIMILAR SOUNDING CALL SIGNS. ANY OR ALL 3 OF THESE POSSIBILITIES MIGHT HAVE AVOIDED THE ERROR.