The call centre is probably based on senority, so the folks with most tenure will have the early shifts. If you want your call answered really fast call in the A.M. These folks are all about getting their AHT down to a minimum so they will want you on, and off the phone within minutes.
Call later in the evening and you'lll probably get someone a little newer, who'll take the time to solve your probably but might require more holds, however they might be nicer and won't care if their AHT is a bit higher.
*AHT - Actual Handle Time.