Seems that the AA Callback route suggested by QBZappy is not where you want to go.
I think OBi110 can do what you want but you have to rely on the Auto Attendant (AA).
When the call comes in from VOIP or PSTN, you can setup the unit to invoke the
AA (for certain caller numbers that you want, or for any caller number esp. if you don't
have caller-id).
The AA provides 3 options: 1, 2, and 3. For your case, you can have your friends dial
2 immediately, then enter a PIN followed by #, and then the target number followed by # to
call further (VOIP or PSTN, etc.).
If the caller does not select any of the 3 options, the AA will announce the menu 2 times (10-20s), then route the call to the PHONE Port quietly. The caller can also select option 1 to force the
call to ring the PHONE Port immediately also.
There is no option to "silent" the AA unfortunately. But you can ask your
callers to just key ahead the option (1 or 2) to skip the AA announcement.