This has happened to me a couple times in the past two weeks at only one of three sites (SITE 1 below). The same GV number is set up to ring at all three sites. All three sites use an Obi202. All three sites are with Comcast as the service provider. They use different cable modems/gateways or routers so perhaps that is where the difference lies. Site 1 has the oldest equipment:
1 Obi202, Comcast, Comcast Gateway (Netgear CG3000DCR) & router Cisco RV 180W
2 Obi202, Comcast, Comcast Wireless Gateway (Cisco DPC3941B)
3 Obi202, Comcast, Cable Modem (ARRIS SURFoard SB8200)
I will try changing the DNS to 8.8.8.8 and 8.8.4.4 as suggested by Zorlac above.