If you need your phone number every day, the safest order is simple: do not cancel the old line first. Apply for the new service, wait until the new SIM/eSIM and account are ready, then perform the final MNP switchover at a time when you can immediately test calls, SMS and data.
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- The one rule that prevents the biggest mistake
- When should you actually press the final switch button?
- MNP One Stop vs reservation number
- Campaign timing is a separate question from MNP timing
- What happens to the old line when MNP finishes?
- If your real problem is getting a Japanese number, not keeping one
The one rule that prevents the biggest mistake
Keep the current line active until MNP is completed. au states that the old-carrier contract ends when the MNP procedure is implemented, and SoftBank likewise describes MNP as the process that transfers the number to the new carrier. Cancelling the old line yourself before the transfer can put the number at risk.
When should you actually press the final switch button?
Do not trigger the final transfer while the replacement SIM is still in delivery. Keep the old line working until you are ready to insert and test the new SIM.
Make sure the new account is approved and you have the current activation instructions before the final switch. Some carriers separate the line-switch step from downloading the eSIM profile.
If you need SMS for banking, tickets, work or travel that day, schedule the change when you have time to troubleshoot instead of minutes before an important verification code.
MNP One Stop vs reservation number
Japan now has two common routes. With MNP One Stop, supported carrier combinations can start the transfer from the new carrier without first obtaining a reservation number. With the traditional route, you obtain an MNP reservation number from the current carrier and give it to the new carrier.
Official carrier guidance commonly shows a 15-day validity for an MNP reservation number. But application channels can require more remaining validity. For example, au currently says its online shop requires at least 10 days remaining. Always check the destination carrier before requesting the number too early.
Campaign timing is a separate question from MNP timing
The practical order is: 1) identify the campaign conditions, 2) protect the phone number, 3) make sure the new line is ready, 4) perform the switch, 5) complete any post-activation requirement before its deadline.
What happens to the old line when MNP finishes?
Once the MNP transfer is completed, the old carrier contract for that number is normally canceled as part of the process. That is why manually canceling the old line first is usually the wrong order.
If your real problem is getting a Japanese number, not keeping one
If you do not already have a Japanese mobile number to transfer, MNP is not the right tool. You need a new voice/SMS-capable Japanese mobile line instead.
Real Japanese number vs virtual number →
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Official sources: au: MNP preparation and contract timing · au: online line switching / eSIM and SIM flow · SoftBank: MNP switchover
