How can I set the actual number that calls are forwarded to?
Well, unfortunately, you can't. We wish you could, because we get asked this question all the time. But you can't.
If you're wondering what we're talking about (something that happens quite a bit, now that we think about it) Lync Server's voice policies let you determine whether or not your users are allowed to forward their work calls to an alternate phone (such as their home phone or cell phone). By default, users are allowed to forward calls. If you'd prefer that they not be allowed to do this then all you have to do is disable call forward in the appropriate voice policy. For example, this command disables call forwarding at the global scope:
Set-CsVoicePolicy –Identity global –AllowCallForwarding $False
That part's easy. However, what people really want to do is be able to specify the phone number that calls can be forwarded to. For example, suppose you want Ken Myer to have his calls forwarded to (425) 555-1219. Can you run some PowerShell command that will cause any calls to Ken's work number to automatically be forwarded to (425) 555-1219? As far as we know, no, you can't.
Hey, you win a few, you lose a few, right?