It depends on what carrier the ketai belongs to and what type of service it used.If it was a softbank prepaid and the number hasn't expired then you can just purchase phone credit and continue using it. If the number expired then it won't be possible, however.
If it is was a contract phone then it may be possible to get a new contract and number with the carrier that it previously operated on. I tried to do this the other day at au, and the au store said that it was technically feasible, but no one had ever tried to do it before. They said that it would require a deposit of something like 20,000 yen to go with a plan that didn't require a contract, or I would have to sign a 2 year contract. In the end it was quite a hassle, and I went with a softbank prepaid as it served my needs better and for cheaper.
You mention in another thread that you are an exchange student for 7 months? Then perhaps a softbank prepaid would be best for you. The handsets start at 5000 yen and credit can be bought in increments of 3000 and 5000 yen.
