Silent incoming calls
 

I've got some more silent/failure type calls from work to home via my DIDs. Today I tried a few test calls from work to home and the following happened:
#1: 9084XXXX - rang six times and then at the point I should have heard the voice mail message I heard nothing (failure appeared to be a silent call)
#2: 9084XXXX - rang six times and then at the point I should have heard the voice mail message I heard a two tone busy signal (failure appeared to be a lost connection)
#1: 8212XXXX - rang six times and then went to voice mail (all ok)
#1: 9084XXXX - rang six times and then went to voice mail (all ok)

9084XXXX - AstraTEL supplied DID
8212XXXX - SIPME supplied DID

I'm ASSUMING that since the call diverts to voicemail, the problem does not have anything to do with my ATA. Is that a fair assumption?

Thus, the problem seems to be with AstraTEL, Voxalot, or with the interaction between AstraTEL and Voxalot (the switching from signaling the ATA to connecting the call to the Voxalot voicemail system). Is that a fair assumption?

How can one debug this problem?