I just found out that if you're a Vonage customer they have a no-extra-charge FAX sending service:  https://v-fax.vonage.com/

I guess it charges the FAX minutes to your phone plans minutes, if the destination number isn't part of your unlimited plan area.  Anyway, this seems to be a natural part of 'digital telephony', so I'd guess that other VoIP plans might do the same.