It's late, and I don't feel like trying to find where I might have used the word "fee", so I don't know what you read.
There are two main types of call forwarding:
- UNconditional Call Forwarding, also known as "Immediate call forwarding": when a phone number with Unconditional Call Forwarding enabled is called, that carrier will subsequently forward ALL calls, immediately, to whatever other number you programmed it to use. The phone number with forwarding enabled will generally ring once, to let you know that a call came in and was forwarded (a reminder, since otherwise, you'd have no idea why the phone isn't ringing). Most carriers charge a per-minute rate for this, since they are carrying both legs of the call, and don't want their numbers used for this for free.
- Conditional Call Forwarding, also known as "no answer/busy transfer": inbound calls to the number with CCF enabled will ring normally if you're not on a call and you answer the phone within the ring period. Otherwise, the call will be forwarded to the number you programmed it to use. Some carriers offer this included with their normal monthly charge. Typically, these are the "big four" mobile carriers.
Other carriers charge an extra monthly fee to provide the service, or don't offer it at all.
Bottom line: if you have mobile phone service as a direct customer of one of the big four mobile carriers (NOT via one of their MVNO resellers), then CCF is included. If your mobile service is via a MVNO (e.g. StraightTalk, Tracfone, Ting, MintSIM, Lycamobile, etc), then they typically don't offer the service, or they only offer it with some of their carrier partners, but not others. For example, Red Pocket offers CCF on their "GSMA" (AT&T) plans, but not on their T-Mobile plans. If your mobile service is a wholly-owned subsidiary brand of one of the big four, such as Boost Mobile, Virgin, Metro by T-Mobile, then they may or may not offer CCF.
If you are having trouble understanding this, then contact your service provider and ask them.