Will the fax know if it's a phone call or a fax?

There is no mystery, seriously. You send a fax exactly the same way as you make a phone call; instead of talking to a person, your fax machine talks to another fax machine. Clicks, squeals, buzz sounds.

If it says it's busy, well, it's busy
If it doesn't answer, it's not set to AUTO RECEIVE
If it says OFF HOOK another device is using YOUR phone line.
If you don't hear a dial tone when you press the HOOK or MONITOR key, the phone line is dead. Check the line with a regular phone.

If a dial tone sounds weird, it is a DIGITAL phone line. You need an ANALOG or regular phone line.

ALL FAX MACHINES ARE ANALOG DEVICES AND WILL NOT WORK RELIABLY ON A DIGITAL SYSTEM OR A COMPUTER....Magic Jack, Vonage, etc. are DIGITAL PHONE SYSTEMS. Don't believe what the Vendor tells you. They are for designed for VOICE GRADE PHONE CALLS ONLY.

The settings in your fax machine were designed by people who are a lot smarter and make more money than us. DON"T MESS WITH THE PARAMETERS.

It's all that SIMPLE !!