When I first arrived in TW, I paid for a T-Star 3 month SIM in cash. The 3 months have come to an end and although I have used up my minutes, I still have LTE+ data everyday. There’s no way T-Star can charge me anymore, did I just get lucky?
Likely the card had a load of money on it separate from the minutes and perhaps your data usage is being deducted from that.
Easy way to check is to check your balance after using some data. If it drops then you’re still paying. Usually it will drop to $0 and then the data will disconnect.
If it’s like in the U.S., even when a telecom service is cancelled and no new SIM card is inserted into the cell phone, the older (inactive) telecom provider’s name will still show up in the top-left corner. If you tried to make a call, you’d get one of those recorded messages stating something about service over, or whatnot.
So, try and use that old SIM to call your other cell #, or someone who is in same room as you and see what happens. Obviously, don’t answer phone if actually it still is active, ha ha, cuz you may have minutes left. Who knows.