I got caught out when Switching my mobile number to iD from Plusnet, a process also known as Porting. On the switching form, don’t enter the Transaction number from your initial purchase. That one begins (in my case) CLC- and is followed by 9 digits. It won’t work for switching.
Instead, look in your Welcome email for a ‘Transaction number for switching’ that’s also of 9 digits but without a prefix. Mine begins with ‘4’ and I’m told this is usual. Put this into ‘Transaction number’ on the ‘Switch to iD Mobile’ form, along with the PAC Code and the relevant mobile numbers, etc, which all need to be exactly right.
I hope this helps someone.