The problem with Tracphones is that they don't really have roaming per se. They (Tracphone) have contracts with all the service providers and buy huge blocks of minutes at an enormous discount that they then re-sell as regular minutes that the customer buys for the phone. I could purchase a Tracphone in Kalamazoo, drive to Denver before ever turning it on and go through the activation process using the service of a local Denver carrier. This would not show up as the phone roaming in Denver, nor would it show as the phone being "home" if it was taken back to Kalamzoo. There is not really a "home" market for Tracphones, they just work as long as you buy minutes for them and as long as there is compatible service in the area.
The system as a whole has to know how many minutes the phone has. Thus in one or the other manner there is a central point of contact.