C560 FLT CREW EXPERIENCED TCASII RA DURING CIRCLE TO LAND AT DPA.

Narrative

ATIS AT DPA WAS RPTING RWYS 10; 15 AND 2L IN USE. APCH PUT US ON VECTORS FOR LOC RWY 10 APCH; GS WAS OTS. WE WERE THEN CLRED FOR THE APCH; TOLD TO MAINTAIN 2600 FT UNTIL ESTABLISHED. APCH CALLED US FOR A TA AND WE TOLD THEM WE WERE IMC. A SECOND TA WAS GIVEN; WE RPTED STILL IMC. APCH THEN TOLD US TO CONTACT TWR. WE CROSSED THE FINAL APCH FIX AND BEGAN OUR DSCNT. WE ATTEMPTED 2 INITIAL CALLS TO TWR WITH NO RESPONSE. AT THIS POINT WE RECEIVED AN RA ON OUR TCASII AND RESPONDED BY LEVELING OFF AT 1800 FT MSL; WE HAD BROKEN OUT OF THE CLOUDS AT APPROX 2200 FT MSL. WE THEN CAME WITHIN 200 FT OF SUBJECT TFC. ON OUR THIRD CALL TWR INFORMED US TO TURN L AND AFTER R BASE FOR RWY 20; THEN CALLED BACK AND ASKED IF WE COULD STILL MAKE RWY 10. WE SAID NO AS WE WERE TOO HIGH AND HAD BEGUN OUR L TURN TO ENTER R BASE FOR RWY 20. WE WERE NEVER GIVEN ANY COURSE CHANGES TO AVOID TFC BY EITHER APCH CTL OR DPA TWR. ONCE ON THE GND; CONTACT WAS MADE WITH TWR OPERATOR VIA LAND LINE. HE INFORMED ME THAT 1) I SHOULD HAVE BEEN CLRED FOR LOC RWY 10 CIRCLE TO LAND RWY 20; 2) THE TWR WAS UNDERSTAFFED; ONLY 2 CTLRS IN THE TWR; 3) ZAU WAS ON THE LAND LINE AND HE COULD NOT HEAR US MAKING OUR RADIO CALLS. I TOLD HIM THAT I WAS CLRED FOR A STRAIGHT-IN LOC RWY 10 APCH AND; EVEN IF I WAS CLRED FOR THE CIRCLE TO LAND; I NEVER REACHED MY MINIMUM DSCNT ALT AND HE ALLOWED THIS OTHER ACFT INTO MY CTLED AIRSPACE.