While wireless or cell phone carriers do not support porting for toll-free numbers, there are a variety of landline and VoIP carriers that do. This includes NumberBarn and NumberGarage.
It's always best to check with the gaining carrier (i.e. your new carrier) to determine if they support the number.
Finally, if you really need calls for your toll-free number to route to a cell phone, consider using call forwarding to a local cell phone number.