my observation is based on my practical experience.
i) I got many complains from those sites regarding the problem of Incoming Calls.
ii) I had personally tested and found that the problem is there. Whenever i call to someone at those sites, most of the time(60-70%) I got the “Not Reachable” announcement. But at that very moment they could call me without much difficulties.
NB- We use SIGDEL-LONG, TEI-40