I have been trying the SIP for Skype beta with IP Office 500. It works pretty well for the most part, but I have found a few annoying quirks that would keep me from recommending it. Since this is my first time using a SIP trunk I don't know if these issues are common to other/all SIP trunks.
#1 - On outgoing calls I don't hear the first couple of seconds of audio when the receiving party answers the call. I usually miss the first three or four words that they say. (Incoming calls work perfectly)
#2 - When calling a number that is busy, for the first few seconds you hear a normal ring and then it switches to a busy signal.
#3 - A TAPI app that I am running that provides call control frequently has problems dealing with calls that are on the SIP trunk. These errors don't seem to occur on a regular PSTN line.
Other than that the trunk has performed really well. The sound quality has ranged from good to excellent and I rarely experienced any hiccups.
I also realized that I had some extra flexibility assigning DIDs. At first I purchased a DID via the web interface and assigned it to my SIP trunk. Then after testing an inbound call I moved the same DID to a business Skype user account. I was then able to log in with Skype on my iPhone with the credentials of the same business user account and receive inbound calls on that DID. Within seconds I was able to switch it back and receive calls on the SIP trunk again. I'm not sure how often something like that would get used, but it seemed kind of cool to have that flexibility. Maybe it would be useful when dealing with an outage or something.
Quin