[agwa]

Same way - fax numbers are no different from regular phone numbers in the PSTN. (That said, once VoIP enters the equation, you want to use a fax-friendly codec like T.38.) HelloFax is surely not their own CLEC so they're buying phone numbers (with a monthly cost) from one or more CLECs or CLEC resellers. If you know a HelloFax phone number you can look up who the CLEC is using this form:

http://www.alcazarnetworks.com/data_services_lnp_lrn.php