You can call forward to a pots phone on an FXS port if you set up a dial-peer for the port with a destination pattern.
Here is an example:
ephone-dn 8 dual-line
call-forward noan 5000 timeout 20
telephony-service
call-forward pattern .T
dial-peer voice 10 pots
destination-pattern 5000
port 0/1/0
SIP call forwarding is supported only on e-phones(IP phones) not on the phones configured on the gateway. (FXS).