First of all, you need to ensure that your provider supports faxing. Without the provider giving this support whatever you do is pretty much futile.
Next, in the 3CX Fax Server settings you will need to have a fax enabled DID forwarding to this.
To test this number you will need to send a fax, not call it. You will not get a fax tone.
Alternatively, you do have the capability of connecting a fax machine through an FXS gateway
Create the Fax Extension first, then go and assign it on an FXS gateway as an extension
A 3rd option is to have a DID assigned to a user, where the fax will be received and forwarded to their email. This would be a Fax Forward in the Fax configuration.
Again, do not test this by calling the fax extension(s) and expecting a fax tone, unless you are calling the DId of a physical FAX machine.