"Switch your number and we'll give you ¥20,000" feels like free money. Mio doesn't feel — it calculates. The deal is engineered so the headline number is the one thing you remember and the total cost is the one thing you don't. Here's the math.
MNP (Mobile Number Portability) means you keep your phone number when you switch carriers. Shops and carriers pay to acquire switchers, so they dangle a big cashback (say ¥20,000) or a near-free handset (a "¥1 phone"). None of that is charity — it's a customer-acquisition cost they expect to recover from you over the months you stay. The recovery mechanism is usually a more expensive base plan plus a minimum-usage period (the lock-in).
1. The lock-in plan is expensive. The cashback is tied to a plan costing far more per month than an MVNO. Over 12 months the plan difference alone can exceed the cashback.
2. Cashback arrives late — or as points. It's often paid months later, sometimes as carrier/shop points rather than cash. Points expire and can only be spent in one place, so a "¥20,000" point payout is worth less than ¥20,000 cash.
3. Paid option add-ons you must cancel. Deals frequently require you to sign up for extra options (insurance, streaming, support). Forget to cancel and the monthly fees quietly eat the benefit.
4. Minimum-usage / early-exit penalty. Leave before the minimum period and you may lose the cashback or owe the handset balance. The lock-in is what makes the expensive plan stick.
Compare a cashback deal on an expensive plan against a plain MVNO, using an existing phone (no new handset) so it's an apples-to-apples plan comparison.
| Over 12 months | Cashback deal | Plain MVNO |
|---|---|---|
| Base plan / month | ¥4,000 | ¥1,500 |
| Plan × 12 months | ¥48,000 | ¥18,000 |
| Required option (must remember to cancel) | ¥500 × 6 = ¥3,000 | ¥0 |
| Cashback | − ¥20,000 | ¥0 |
| Total cost, 12 months | ¥31,000 | ¥18,000 |
Even with the ¥20,000 back, the cashback deal costs roughly ¥13,000 more over the year here. Stretch to 24 months and the gap widens further, because the cashback is one-time but the plan difference repeats every month.
| Over 24 months | Cashback deal | Plain MVNO |
|---|---|---|
| Plan × 24 months | ¥96,000 | ¥36,000 |
| Cashback (one-time) | − ¥20,000 | ¥0 |
| Total cost, 24 months | ¥76,000 | ¥36,000 |
The cashback is a fixed ¥20,000; the plan gap is ¥2,500 every month. Time is on the MVNO's side.
It's not always a trap. Run the TCO and the deal can win if: you genuinely need a new handset and the "¥1 phone" plus cashback beats buying the device outright; the required plan is close to MVNO pricing; there are no paid options or you'll reliably cancel them on day one; and you can exit at the lock-in end and re-switch. If any of those fail, the plain MVNO usually wins.
To do MNP you first get an MNP reservation number from your current carrier (online, app, or shop), then use it at the new carrier within its validity window — this is what keeps your number. You'll need ID (residence card / My Number, depending on carrier). Contracts, cancellation terms, and option opt-outs are usually written in Japanese; the exact cancel-by date for paid options is the detail that costs people money, so confirm it in writing. If a credit card is the blocker, some MVNOs allow debit or bank transfer — see the no-credit-card SIM guide.
Ignore the headline. Compute plan × months + device − cashback over the whole lock-in. For most residents who already have a phone, a ¥1,000–2,000/mo MVNO beats a ¥20,000-cashback deal on an expensive plan within a year. Take the cashback only if the full TCO — after converting points to cash and subtracting must-cancel options — actually comes out lower. The meter is monthly; the cashback is once.