mystery.shopping@ipsos.com ... Just kidding, I don't know.
In the past, when I did shops more frequently with this MSC, over time I knew who to contact in terms of the client/project manager. Most of the time, I was able to get my issues resolved. But I didn't do certain types of shops, as well.
In terms of best, I found Bill Carden to be very responsive. The few times I had to contact him.