The most common problem I see here is folks leave the default of "send to greeting for" on the one key rule instead of selecting "attempt transfer for"... can you verify the one key rule on the call handler for this?
If you dial the openeing greeting and then just dial the extension number of the subscriber in question, does their phone ring properly? This would isolate the problem to the one key configuration...
another thing that happens from time to time is the transfer fails immediately (i.e. the TSP comes back and says the extension is busy right away) and it sounds as though we didn't even try the transfer... You can "watch" what the call is doing with the StatusMonitor.exe which you can find in the tools depot or in the commserver directory. It's a little awkward to use but you can monitor all the ports and then watch what happens when you press 1 from the call handler greeting. It should show the call going through PHTransfer (the conversation used for transfer attempts) and then progressing to PHGreeting if the transfer attempt fails.
If that's the case we'll have to look into what's going wrong there.