Voicemaster is still in the process of fixing their problems, I think.
I also have the same issue.
Mine reboots atleast once a day.
According to my experience Voicemaster has many small small isuues.
I was analizing all my droped calls, and I feel like Vociemaster has compatibility issue with many systems.
The rebooting happens because of these issues.
(I know they had problems with MERA softswitch like 2 months ago. Now they fixed it)
Anyway my recommendation is upgarde to the latest version for Voicemaster available and keep upgrading it.