sic698 wrote: on the phone directly ? dialing *72 ?
No. When handled by an SPA ATA, the forwarded call will drop directly to voicemail since FPL rejects 302 SIP responses (redirect invite with temporarily moved) to Invite for call forwarding.
If you look at
viewtopic.php?f=14&t=2337&p=13266, forwarded calls will drop directly to FPL's voicemail system, which you may find useful for dealing with known telemarketing numbers.
Obihai ATAs can bridge calls and have a setting called X_Use302ToCallForward, which should be disabled when using FPL. If you want to forward calls using a SIP device, the SIP device needs to take the incoming or existing call and bridge that call to the destination when using FPL, and the only ATAs I know that can do that are Obihai's.