How to transfer corporate phone number to me

Hello!

I’m trying to transfer phone number from my ex-employer to me, when I left employement they send me a letter confirming that they are releasing the number to me. I’ve received an sms from o2 that my “Portierungserklaerung” is valid till 3rd of August.
I tried calling english support and while they’ve sent me links to instructions on how to do it for private customer contracts, those instructions don’t seem to be applicable to o2 bussiness contracts.

So my question is how do I transfer phone number from o2 bussiness customer to me, considering that I have the current owner written consent.

Thank you!