From the perspective of an international student, the top 3 phone companies in Japan are Docomo, Softbank and Au. I have heard that before all 3 companies offered prepaid phone but apparently many criminals used it for illegal transaction ( I don't know how ) so as of now only Softbank offer the prepaid plan. You must have your gaijin or resident card aka green looking card you received at the airport in order to purchase the phone.
http://mb.softbank.jp/en/prepaid_service/model/ ( Here is a list of different models you can get ) I got the Softbank 740SC one, it was one of the less expensive one. It depends on where you are getting your phone the price ranges from 4000 to 5000 yen but the cool thing is you will get to keep that phone ! You are going to have to get used to the T9 it was quite a challenge for me. Some of you out there might want to bring your phone but the Sim card is completely different that you will not be able to use your phone in Japan. My friend and I brought ours but we ended up using it to take pictures/take notes and use wifi ( rarely since again they don't really exist in Japan .)
How long does it take ?
I was lucky enough to have a Japanese friend with me to do this process it took us 30 minutes. I also accompanied my friends 2 weeks earlier it took us at least 1 hour because our Japanese wasn't that great but the people were very patient and friendly so have no fear if you are not fluent in Japanese.
Prepaid Plan & Costs
http://mb.softbank.jp/en/prepaid_service/unlimited_mail.html [ Unlimited mail information ]
This is the layout of the prepaid plan, it's quite expensive to call one another so most of the time my friends and I text each other. Another thing that is very interesting in Japan is when you open a phone you'll have an email address with that phone and when others text you they usually send it to your email ( mail box on your phone but they can also send to your yahoo, gmail and etc. ) The reason is because they offer an unlimited mail option, my friends and I always text each other using the mail address , rarely do we call or text it to our phone number. This is very convenient at times because when you want to reach your friend, you can send an email to their phone and they can email you back on your gmail account or yahoo account, the mails will stack up in your mailbox though.
Since it is prepaid you can also make international call to your family and relatives but keep in mind it's going to be expensive. I actually had a lot of minutes at the end of my trip because I was always too scare to used up the money in the account so toward the end of the trip I made several calls to my family in the U.S.
I remembered I paid 3000 yen for my phone and 2000 yen for the minutes + unlimited mail ( this plan i really recommend you go apply for because even in the U.S. we text all the time instead of calling anyways). This unlimited plan must be updated every month ( you have to make a call and click number 2 for English I think and then you can figure out how to update it from there. ) Once you're out of minute you don't have to return to a softbank store or where you opened the phone, you can just go to Lawson or 7-eleven to renew it. The staffs at the store would be more than happy to help you.
My Friend & Our Cellphones
That's all the information I can share in terms of getting a cellphone if you have any specific questions about it feel free to send me a message :D