As for the 'inline decoder' for the fax, I don't think that's possible, since fax machines are given regular phone numbers. If they were to start giving fax machines their own special type of numbers you could call, and assign a special new set of frequencies to them. But then you wouldn't need your decoder.
Most folks I know end up having to get a second line, then just use it solely for their fax machine. Of course not everyone wants to pay extra money to have a dedicated line, especially if they don't use their fax machine enough to justify the cost of a dedicated line. Now days fax machines are starting to die out in favor for emails. Plus you can send faxes online now, rather than having to use a fax machine, which is more convenient and doesn't cost anything, so folks are able to totally do away with their fax machines altogether. I personally like fax machines though.