Google Voice is not impacted by SIP ALG, but other VoIP services may be. It was a bad idea to begin with, and it is rarely used, and it just causes trouble, so it's fine to leave it disabled. There is no impact whatsoever to leaving it that way.
I am not going to continue the discussion about your ISP connection, since I've repeatedly pointed out that it is the root cause of the instability of your VoIP experience. You can take your OBiTALK device and phone to some other location (friend's or relative's house with a different/better ISP service) to confirm or dispute this.