You need to have good bandwidth in both directions to make faxing work. To send, precede the fax number with *99 when dialing. This selects a high-bandwidth codec that works better with faxing. There is an option to select this codec as the default on my.ooma.com if the line is only used for faxing. This would not be a good codec to use for voice. so don't select that option if you also make voice calls with that Ooma account.