Don't know if this will be of any help but below is the section of the haelp files from my fax/tad modem software. Associating distinctive rings to the different types of information being sent in
Note: The features described here are not supported by all modems.
You can setup the modem to answer the phone based on the type of information that is being sent. By programming the modem to recognize different ringing patterns you can have a single phone line perform as though it were four.
To do so, you will need to purchase distinctive ringing service from your local telephone service provider. Consult your telephone company for detailed explanations on the different types of distinctive rings available.
If your client is paying for two lines why not separate them.
I am a builder working from home. I have VIOP phone service with TWO phone numbers. The primary is my personal number and the second is business FAX/ Phone. I have a Phone/FAX/TAD modem and software that effectively works. My software does have options for distintive ring, however I am unable to use that feature as I am also using a propietory Panasonic telephone PBX that has distintive ring built into the system. This prohibits me from using the distintive ring feature of the FAX software.
I suspect this feature would otherwise work, but can't say with any certainty.
The name of the software is Classic Phone Tools. You may want to contact them for additional information. They may be able to advise you about which modems will support what features.
FYI, I bought the Modem for $10 and the software was bundled. I have had some issues with the software when upgrading to WinXP but it is working currently. I was advised originally that I needed to upgrade the version of Classic Phone Tools. I would have done that if it had become necessary but an online upgrade solved by problems.
One other thing to note. I recently switched to VIOP from a standard pair of phone lines from AT&T. The sysem works the same with each. The VIOP is just much cheaper. My second line costs $19.95/month.
Hard to state exactly what I pay for my two lines because the services are bundled.
Hope this helps.