Receiving calls that are very close to where you are is actually quite accurate, since usually you are assigned a beat. A beat is a sector that you will patrol and is about the size of some districts in Liverty City, such as East Holland, while Alderney may be cut in 3 sectors. Central Island would be a sector by itself and unless units doing that beat would require assistance, you shouldn't have to go to it, unless you are one of the beat cars in the sector. Most of the time, an unit may be only 5-15 minutes away at normal driving speed, but due to gameplay decisions, it is shorter in LC. The problem that I encounter is when I patrol close to the coast, I may get a callout on the other side of the river and is out of my beat, sometimes the distance can be as ridiculous as from Star Junction to Hove Beach.
Also depending on the callout, code 2 or 3 may not be required, thus making the drive longer time-wise.