Salvaging and porting old cell number

 Hello, 

I saw this question a few times and see that I need to call at&t to see if the number has been reassigned, but I did not see the number to call. Is it 800-288-2020? I used to work for uverse and that was the main number for basically everything back then.

My question is, I had an old cell phone number that was mine for about 15 years. I changed about a year or two ago. If the number is still available, can that old att cell numbers be ported to Google voice? If not, can it be transferred to Google project fi? That is my current cell provider. I know this isn't a Google forum, but it's the same as transferring it to Verizon or sprint or whatever, it's just swapping it with a line that's currently active. I don't know if that's possible. 

If it can't be transferred, and I end up having to just sign up for a new plan, how do I ask for the number back? Don't they usually just assign one from a pool, and aren't allowed to use or search for specific numbers? I hate needing to have 2 cell phones with 2 separate plans, but I will if I have to.

Sorry for so many questions, I just would like to get my old number back. If anyone can help with an 800 number to get started and what to ask for, I'd really appreciate it. 

Thanks