Trac Fone, Straight Talk and NET10 phones will favor their home network and refuse to roam in an area where the home network claims to have coverage.
So if you have a phone whose home network is T-Mobile and you are in an area where T-Mobile has spotty coverage, the phone will refuse to roam on AT&T even if AT&T has a better signal. Which network a given Trac Fone, NET10 or Straight Talk phone is homed on depends on the market it's sold in.
The first six numbers of the ID determine the carrier: 890126 = T-Mobile, 890141 = AT&T, 891480 = Verizon. AT&T Tracfone SIMs have a code on them that includes the sequence SIMC4, like TF64SIMC4.
Non-smartphones with model numbers ending in "C" like the LG 501C and LG 220C all use CDMA technology and are homed on Verizon. These phones home network is Verizon but they will roam on other CDMA networks like US Cellular in areas where Verizon doesn't have coverage.
These phones do not have SIM cards and are incompatible with GSM networks like AT&T and T-Mobile.
Trac Fone and its sister brands NET10, and Straight Talk, do not have their own networks.
They're Mobile Virtual Network Operators (MVNOs) that buy wholesale minutes, texts and data from AT&T, T-Mobile, Verizon and over a dozen smaller regional networks.
Trac Fone companies seem to prefer selling T-Mobile homed phones, followed by Verizon with AT&T as a last resort.