I might be interpreting this wrong as to what you have done already or how you want it to work, but if the Polycom is connected to a SIP trunk and if the link is UP and functional (SIP LINK STATE ALL will show probably) and reacts on the 59800 number would you not:
- Program a route in "ARS Routes" to the SIP trunk (Leave settings default as in no restriction and no digit mod, which is probably 1 in both cases)
- Program the 59800 number in the ARS Digits dialed to that route number you just programmed pointing to your SIP trunk with digits to follow 0?
- Make a DID in the speeddial pointing to the ARS entry corresponding to the number you dial internally and which corresponds to the external number, e.g. Speeddial = 3000 -> Actual number = 59800
- You can then dial the speedcall or ARS entry and you should end up in the Polycom receiving a call through the SIP trunk. You can then also program it in your Teldir but you can not make the number private (just saying)
As said I might be reading this wrong, but this is how I am used to do this, as it works the same with PRI -> PRI conference systems.
Make sure the "trunk attributes" you programmed for the SIP trunk mention "Dial In trunks Incoming Digit Mod = 0" for the SIP trunk as well, as you don't want any modifications in the most cases.
As said I might be interpreting this wrong but this is how I would do it roughly.
Dutch