I've recently been plagued with the same problem of the adapter un-registering from VOIPo's servers the last couple of months. That you see it too makes me thing it's some new bug between servers/adapters, or prone to fluctuations in ISP service.
FWIW - VOIPo have changed some values in my adapter to address this issue, though that testing is ongoing. The items changed were the "registration cycle" and/or the "Registration Expiration."
Might be worth having VOIPo try those to see if they help you.
Dave -- Cox Preferred HSI 12/5~SB6120~Grandstrem HT502~FVS318G~WAP54G OTA (...and proud of it!) VOIPo