if the phone is sim free yes you can put any sim in it
the number on the old 121 sim card will probably not be registered anymore as 121 are now t mobile and have been for a while.
you could however just put it in to try and see what happens. on the nokia 1112 i think if you go to the phone book and scroll down it will say my number which if you choose it should give you the number of the sim card. once you have it you could dial it from another phone and if it rings it will still be working! if its working the best thing to do is to go to a shop and buy a tmobile top up and follow the instructions as to how to top it up.
if the number is not working you can buy sim cards for about �10 in shops about �5 on markets and about �2 including postage on ebay. you can get free sim cards if you look about but that is probably gonna take about 28 days to get to you.
hope that was gentle enough