I just got the impression from this post that RobertoS was saying you should complain to the gaining provider...
The notice from the gaining one allows you to cancel for any reason, without penalty. So acts as both a cooling off period and an anti-slamming/false order defence.
The losing provider can (basically) only cancel the migration if the gaining one refuses the user's cancellation request. I forget the exact wording.
I didn't think it worked like that, given that you may not know who the gaining provider is, if you get my drift.
We are basically talking at cross purposes

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The Gaining Provider Led (GPL) system is designed on the basis of genuine migrations, and is as I described in the full post.
The cases where the losing provider can intervene are quite restricted, but the conversation had veered to a outlying happening.
General Condition C7 and Appendix 1 lay out the standard procedure and what to do when things go wrong.
The rules are not basically designed to prevent slamming, but do cater for it. They are designed to make migrations as easy and painless as possible for the user

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