This happens to me about once a week, it is totally random, I do not reboot my obi unless I upgrade the firmware. It does not seem like it is related to how long the device has been running. In fact I am not even sure if this is a Google chat problem or obi problem. Whenever this happen, I always report it to Google (go to Google Voice/call history, find the call history entry, check the "call quality" box with "person cannot hear you", this seems to be a very wild spread problem, hope it get fixed