I've used APRS many times over the years. It should work like this: Ham1 mobile is broadcasting position on APRS with a one or two line message stating the frequencies he is monitoring. Ham2 mobile notes on his computer he is within radio range of ham1, reads message, decides to give ham1 a call. Ham1 notes ham2's location on his computer turns up his receiver volume and answers the call.
This is an old thread I know but the above is 90% right. The missing element is your transmitter is set to a PL of 100.0. If you are close enough to get the packet and decode it directly (important part) you should be within mobile range and be able to hail them on Simplex on 144.39 using PL of 100. No need to adjust a volume knob. Just make contact and QSY to another frequency of convenience. A lot of times I'll have them go to 440 because it won't interfere with the APRS tx.
I have done this in the past and it worked FB. Worked several folks this way and had a nice surprise chat. Better than calling out on 52.