First, to acquire the number, you need to visit a shop of the provider you are moving away from. If you are on a normal contract, the process will be fast and painless; you'll have an MNP Reservation in a few minutes, and you won't have to pay anything up front. In fact, you won't have to pay anything at all until you use the number. When you do, the charge for the number (usually 2100 yen) will appear on your final bill from the provider you are moving away from. (If you haven't already, make sure you know what kind of [LINK]Early Termination of Contract fees, if any, will apply when you move away from your current provider.)
If you have a [LINK]prepaid phone, things are a little more complicated. Actually, prepaid users can only get an MNP Reservation from a cellphone shop. That means you can't get it online or by calling. It takes about 30 minutes to an hour to process a prepaid cellphone request, since the local shop will have to call the provider headquarters in Tokyo; the request will be processed in the head office, and the time it takes will vary according to the availability of a support technician to process your request.
Also, since prepaid customers don't receive bills, you'll have to pay for the MNP Reservation up front and in cash. If you end up not using the MNP Reservation Number and/or it expires, it will take time and effort to get that money back. (You'll have to visit the exact same shop you made the request at, file a claim for the money, then wait again while that is processed. If you let the MNP Reservation expire, it is up to the shop whether they will allow you to reclaim the money or not.)
You noticed me say "expire" in that last paragraph, right? MNP Reservations are only valid for 15 days from the date of your request. If you don't use the reservation within that time period, it becomes invalid and you'll have to request another.
Once you have the reservation (a long string of digits), just show it to the new provider you want to sign up with. The easiest (and best deal) is to just sign up for a brand new contract with a brand new handset. Most handsets are nearly free when you sign up with MNP, and there is no charge at the new provider for using an MNP Reservation during signup.
For those interested, here are the applicable phone numbers and web portals for requesting MNP Reservation Numbers outside a shop:
- Telephone: 0077-75470 (9:00-20:00)
- EZweb: [TOP]→[auお客さまサポート]→[申し込む／変更する]→[携帯電話番号ポータビリティ(MNP)] (9:00-21:30)
- Telephone: 151 (from a Docomo cell) (9:00-20:00)
- Telephone: 0120-800-000 (9:00-20:00)
- i-mode: 各種お申込・お手続き iMenu→お客様サポート→ お申込・お手続き→各種お申込・お手続き→手続き開始（ログイン）→ネットワーク暗証番号入力→各種手続き→携帯電話番号ポータビリティ予約
- Internet: My docomo（http://www.mydocomo.com/）→ ログイン→ 各種お申込・お手続き→ 携帯電話番号ポータビリティ予約
- Telephone: *5533 (from a Softbank cell) (9:00-20:00)
- Telephone: 0800-100-5533 (9:00-20:00)
- Yahoo! Keitai: [TOP]→[設定・申込（My SoftBank）]→[各種変更手続き]→[MNP予約関連手続き] (9:00-21:30)