I get both MWI (the stutter tone) and VMWI (the blinking LED) with both Google Voice on SP1 and Callcentric on SP2, using the settings shown in the attached screenshot (OBiTALK expert config, PHONE interface, Calling Features section).
I also modified the MWI subscribe parms under ITSP Profile A SIP during my troubleshooting, but I don't know if this was responsible for fixing it or not; you can try it if the other settings don't do the trick for both turning ON and OFF the indicators.
With GV, the indicators come on within a minute of a message being left, and go out within a minute or two (this seems to vary, and there were several GV bugs that confused things, now fixed).
You probably know this, but support also depends on your brand/model of telephone, and any settings it has with regards to operating the message waiting indicator.
