There is no way to do it without an unlocked cell phone.
A more accurate way to explain: You don't need an unlocked phone IF
you have a Tmo phn and use a Tmo sim, you have an ATT phn and use an ATT sim, etc etc
the reason to have an unlocked phn is if you want to use an ATT sim in a Tmo phn and vice versa
If you have a friend with a spare flip phn for example that would loan it to you for a few days, then your only cost would be for the sim card which have ranged in price from free to $9.99 recently.