ALTDEV ALT EXCURSION DUE TO A TCASII RA ON UNKNOWN VFR TFC IN AREA.

Narrative

DURING A PHX-TUS FLT WE WERE IN LEVEL CRUISE AT 17000 WHEN WE HAD A TCASII ADVISORY OF TFC AT 10-11 O'CLOCK AND 400 BELOW. THE CAPT INITIATED AN IMMEDIATE CLB AND I ADVISED ATC IMMEDIATELY THAT WE WERE IN A CLB FOR CONFLICTING TFC. THE FREQ WAS BUSY AND IT WAS NECESSARY TO CALL AT LEAST TWICE TO ENSURE THAT WE WERE ACKNOWLEDGED. WE CLIMBED APPROX 1000 FT IN A L TURN TO KEEP THE TFC IN SIGHT; WHICH WAS A TWIN OTTER PARACHUTE JUMP ACFT. WE GRADUALLY RETURNED TO OUR ASSIGNED ALT; DEP SAID THEY SAW THE TFC ON RADAR AT 500 BELOW OUR ASSIGNED ALT. WE LATER WERE REQUESTED TO CORRECT OUR HDG AS THE TURN WE HAD USED TO KEEP THE ACFT IN SIGHT HAD DIVERTED US OFF COURSE. I ASSUME THE JUMP PLANE HAD BEEN WORKING THAT AREA FOR SOME TIME THAT DAY AS THIS IS AN ACTIVE JUMP ZONE; BUT AT OUR CRUISE SPD; 500 FT IS NOT ENOUGH SEPARATION TO BE SAFE; AND THE TCASII PERFORMED AND DEMANDED AN IMMEDIATE CLB AS IT IS SUPPOSED TO. WE SHOULD HAVE BEEN WARNED ABOUT THE TFC AND/OR THE JUMP ACFT KEPT TO A LOWER ALT.