I am not that knowledgeable about the SPA232D, however it appears in many respects to be similiar to the SPA3102 which was an adapter that preceeded the SPA232D and which I am more familiar. I did not find a guide to the FXO port settings other than the SPA232D Administration Guide which has a fairly detailed description of the various settings.
Call forwarding an incoming PSTN line call to a number over your voip network should be fairly straight forward. Assuming the SPA232D works like the SPA3102 you setup a dial plan with your forward-to number and specify the voip account to make the call. There are some settings that affect any delay you wish before answering the call.
The PSTN Tab has a section to setup a voip account. You can use this account for the forwarding or use a gateway account you setup on the Line 1 Tab.
The key settings would be
PSTN-To-VoIP Gateway Enable: Yes
PSTN Ring Thru Line 1: No (Unless you want the phone attached to the SPA232D to ring before you call forward)
PSTN Caller Default DP: N (where N = the dial plan number you setup with the call forward number)
PSTN Answer Delay: N (number of seconds answer delay to capture the incoming caller id if you want to use it)
Dial Plan N(1-8): Setup one of the 8 dial plans that you have specified as the PSTN Caller Default DP for the forwarding number.For Example: (S0<:15101231234>)
Where S0 (S zero) means dial immediately 15101231234 on the VoIP account setup on the PSTN Line Tab
To use a gateway account setup on the Line1 tab instead of the VoIP account setup on the PSTN Line Tab you would add @gw2 to use gateway 2
For your question about enable/disabling the forwarding I believe you would need to change the configuration, i.e. disable the PSTN-to-VoIP Gateway setting.
