If, after taking QBZappy's advice, you still have trouble faxing, you might try another (not free) VoIP service, e.g. Callcentric. They could also serve as a general backup for GV and if desired, provide real 911 service at $1.50/mo.
Alternatively, try a service such as FaxZero (free service is limited to two three-page faxes per day and includes an ad on the cover page).