Well, MOST of the world has cheap (below $10 USD) prepaid plans like europe, I think japan is an exception where "cheap" calls using prepaid sim cards is very difficult (nearly impossible) to get because the three most popular providers docomo, au, Softbank all have very steep prices for just calls. The situation is even worse for prepaid data plans (~$25/mb using Softbank, insane. Texts were also $2.50/text, so that's not an option either). Their reasoning is to prevent criminals from buying burners. And if your phone is locked then you would have no choice but to rent their phones for about $50/week or $200/month, to put it into a monthly contract perspective.

There are a few exceptions such as buying from other networks besides the big three but those networks don't work in non-metropolitan areas (i.e. outside of tokyo, kyoto, or most major cities), and that's when you might need your phone the most, especially if you can't speak Japanese.

The loopholes: using wifi to gvoice call internationally at $0.11/min in Japan, or payphones, because receiving calls is free. I digress, but to explain the situation, I found a store (Mobal) at narita int'l airport that gave free SIM cards where you "pay as you go" at around $2.50/min (the other stores charge ~$1.50/min but you needed to pay $25 flat fee and then a rate of $1.50/min), but I also bought a portable wifi router so that I could make internet calls through gvoice for $0.11/min (or free calls back home in the U.S.) and you can avoid paying them as long as you can make wifi calls (free wifi isn't as common in japan as everyone thinks).