You will have to open a support ticket for this one. (
https://support.fongo.com/hc/en-us/requests/new ) I know it has been possible for people to get their numbers back in the past but it is done at an account level and not really something we can help with here.
This page might help
https://support.fongo.com/hc/en-us/arti ... ne-number-I would follow that page and in the box explain what happened and enter the phone number that you used to have. If nothing else you have created a ticket that is on the right subject and you should be able to get something sorted.
If the number has already been reassigned, it might not be possible to get it back, but the fact you get an error when calling it is a good sign.
You probably want to have a read through this page also
https://support.fongo.com/hc/en-us/arti ... t-deleted-Basically what they are trying to avoid is having dormant numbers in the system as these don't make them any money.
If the number is very important to you, I would suggest moving it to FPL and buying the unlock key. This way you do not have to worry about it ever get reclaimed. There is a bit of initial outlay, so it will depend on how important the number is to you.