I was a Support Engineer for a VoIP Digital & Analogue Gateway Manufacture. One of my many functions was interop testing with top VoIP Products.
Main focus of the testing should be customer based. (i.e. what products will your customers be using if no already using).
if this does not apply then look at what type of application you are trying to impliment and then test products associated to that application.
I hope this helps.
cheers,
Vurcos