My mother has noticed that her VOIP calls drop after 29 minutes. We thought at first it was only when the two of us chatted, but it turns out it happens with everyone, local and long distance. I saw in another forum question that this has been a problem for others, but there was no solution posted. Mom recently received a new modem, but this issue has been going on for months before that was replaced in January.
I had a similar situation years ago in a commercial retail setting. It turned out that it was the burglar alarm security system doing it's regular (every 30 minute) check of the phone line. Do you have something similar?