Are Japanese cell phones locked to carriers?

Question:

I have a Softbank phone I got back during time I spent in Japan two years ago. I will be returning to Japan and I am wondering if I can use my Softbank phone with another carrier's prepaid sim card? Or are phones locked to the carrier?

I would get Softbank again but the landlord tells me Softbank gets poor reception at my apartment.

Answer:

Yes Japanese cellphones are locked to their carriers, you can't use them with separate carriers, unfortunately.