RE: SIP ALG, I wouldn't worry about that for now. That setting only causes problems with one-way audio, not disabling the ability to receive GV phone calls.
RE: how to revert back to Chat: Log into Gmail on your Chrome browser. Look on the left side of the Gmail inbox web page, and find your avatar. It is either a generic head and shoulders outline, or a photo or GIF or JPG that you have uploaded to represent yourself. Click it. A drop-down list should appear. It will either say "try the new Hangouts", in which case, do nothing, or it will say "Revert to old Chat", in which case, click that, and then click the "Are you sure" dialog.