Since several users have now reported the same symptoms, it seems reasonable to revert to an older firmware version for testing purposes. Since Google has rolled back its certificate changes (for now), this might work.
First, set the following so your ATA will not automatically update. This will disable OBiTALK so you will need to use the device's internal web server to configure it. You can change these settings back later, after we see whether or not I am correct.
System Management >> Auto Provisioning:Auto Firmware Update >> Method: Disabled
ITSP Provisioning >> Method: Disabled
OBiTalk Provisioning >> Method: Disabled
Voice Services >> OBiTALK Service:Enable: (Unchecked)
Next, install this firmware from July 2017:
http://fw.obihai.com/OBi202-3-1-1-5695EX.fwLet us know how things go.