I do not have Plus and got this to work (somewhat) on my 202.
Under physical interface for Phone 1 I unchecked both boxes and inserted sp2(music@iptel.org) as the number to call in the MOH SERVICE NUMBER field. I changed nothing else. This URI answers with music.
I called into my 202 from my cellphone, answered the call then placed the call on hold.
If I use the ATT single line phone from Walmart, when I press HOLD, the call was placed on hold with no music. If instead I press FLASH, music appears to the caller and on my phone I got a second dialtone.
I then hooked up my Panasonic cordless base station. I again called in and this time answered the call with my remote handset (I didn't try the base). When I placed the call on HOLD no music, but when I pressed FLASH, again I got music.
Its seems the HOLD code is not a trigger but FLASK is.