I am having the same symptoms on a Obi 202. Was working fine the Sunday before xmas, and during the following week it stopped working. I use it between my Obi in the US and my parents in the UK.
GV works fine, the Obi echo test number works fine, Obi to Obi does not.
Rebooted Obi.
On my syslog server I see Obi202: SIP_TS:Retx, next in 100 after Call Control initiates the new call , then it backs off for 200, then 400 then 800 then 1600 and finally an annunciator tells me the there was no response from the Service Provider for **9 etc.
Maybe Polycom are playing around with DNS on the proxy servers as this looks like it is just unable to resolve the other endpoint