Understanding the mobile number?

I have a cell phone from Huawei and a contract from WinSim. I wrote to a specific contact on this cell phone. The last time probably 2 years ago. I reset the cell phone half a year ago. Whatsapp backups also deleted - both on the device and in the cloud.

I now have a new cell phone with a new contract.

But I still have my old cell phone with the old contract. Is there a possibility to restore the former deleted accounts. Is there a company that can do that? Can I ask WhatsApp to tell my WA contacts to tell me the number from back then.

and second arrived my old contact writes on my old number again on whasapp - would the message get to me on my old cellphone on whatsapp or still because I have not saved the contact anymore? (i.e. Arrived in the last six months where I I still used the cell phone my contact would have written to me - if I open the cell phone again after six months - would I have the message on my cell phone?)

As far as I know (don't use WhatsApp yourself) the contacts are only saved locally on your device, so WhatsApp should theoretically not have your contacts at all.
However, your contacts may have been synchronized with your Google account, maybe they are still there.

There are definitely ways to recover data, but it will likely be very expensive and there are many black sheep in the data recovery sector.

To your second question:

WhatsApp messages are cached on servers until they are delivered. If a message can't be delivered, it will be deleted after 30 days.