I would remove the sim, and clean it with a dry microfiber cloth and re-insert it. Also try restarting the phone after reinserting it. Just rub the metal contacts on the SIM card until the dull metal looks shiny again. It may just not be making good electrical contact. Don't put any water or liquids on it, just use a dry microfiber cloth to clean it. If this doesn't work, the SIM card could be damaged, or the phone could have hardware damage. Do you have another SIM card to test with your phone. If the SIM card is damaged, you can get a new one replaced from your cellular provider.

Or, if you really want to unlock the phone, ask tracfone how many months of service are left on the 12 month contract before they will unlock it, and you could switch to tracfone and keep your same phone number by having them port it from Mint to Tracfone. Then just stay on trackfone for how many months is required to have them unlock it. Or, just sell it and mention to the buyer and in the add that it's SIM-locked to tracfone for "X" number of months. A tracfone user may buy it. This is why I never buy used phones.

There could be an app running in the background that is stuck, or crashed. Sometimes my Map app, or a game running can cause my phone to heat up. When my phone heats up, I force close all the open apps, so that they all close and none are running. This always makes my phone cool again. Also, It's good to restart the phone every few days to keep it running smoothly. Or, if you are in an area with poor cell reception, like 1 bar, the phone could heat up as it transmits at higher power to the cell towers to maintain a connection. Or, if you recently installed the latest iOS update, your phone will heat up for a day or two while it re-indexes all the files, and updates the phone. Under Settings --> battery, you can see what app is draining your phone the most.