EMB145 INBOUND FOR LGA DSNDS WITHOUT ATC CLRNC; SIMILAR CALL SIGNS.

Narrative

WE WERE ENRTE FROM DAY TO LGA. WE HAD BEEN ON THE MILTON 3 ARR INTO LGA; IN IMC CONDITIONS. WE HAD CHANGED FREQS AND WERE TOLD TO DSND TO 9000 FT MSL; JUST AFTER WE HAD PASSED THE BENTY INTXN. THE CTLR VECTORED US TO A HEADING OF 130 DEGS FOR SEQUENCING INTO LGA. SHE CALLED AND TOLD FLT ABCD; 'DSND TO 7000 FT MSL.' WE HAD NOT BEEN TOLD OF ANY SIMILAR FLT NUMBERS BEING ON THE FREQ AND CALLED TO CONFIRM THAT IT WAS 'FLT AECD DOWN TO 7000 FT MSL;' BUT DID NOT GET A RESPONSE AND ASSUMED IT WAS DUE TO HER WORKLOAD. WE REACHED 7000 FT AND WERE FOLLOWING HER VECTORS. WHEN SHE GAVE APCH INSTRUCTIONS TO FLT ABCD; WE STILL HAD NOT BEEN INFORMED THAT THEY WERE ON THE FREQ. BEFORE WE COULD RESPOND TO CORRECT THE CTLR THAT OUR FLT NUMBER WAS IN FACT AECD; FLT ABCD ACCEPTED THE INSTRUCTIONS TO JOIN THE APCH TO RWY 4 IN LGA. AS SOON AS WE REALIZED THE ERROR; WE CONTACTED APCH AND NOTIFIED THEM OF OUR ALT AND BEGAN TO CLB BACK TO 9000 FT. HER ONLY RESPONSE WAS 'I KNOW;' WITHOUT ANY INSTRUCTION ON WHAT ALT SHE WANTED US TO RETURN TO. AS WE WERE PASSING THROUGH ABOUT 7500 FT MSL; SHE INSTRUCTED US TO RETURN TO 7000 FT. ROUGHLY 20-30 SECONDS (SEEMED A LOT LONGER). WE RETURNED TO 7000 FT MSL AND WERE HANDED OFF TO 2 MORE FREQS BEFORE ANY CTLR NOTIFIED US THAT WE WERE FOLLOWING CHQ FLT #ABCD INTO LGA AND TO BE AWARE OF SIMILAR CALL SIGNS. WE CONTINUED THE FLT WITHOUT INCIDENT.