Where you might run into issues is the REN. The Linksys SPA devices, for example, only have a REN of 3. In order to ensure there is enough ring current generated by the ATA to ring all the phones in your house, you will need to add up the REN number for all of your phones.
Prior to connecting your ATA to house wiring, ensure that your house is disconnected from wires provided by the telephone company. Even if the wires do not contain any active service, the voltage coming through those lines can and will fry your device.
Please consult our
whole-house wiring how-to (Distribute VoIP Throughout a Home) for more information.
Connecting multiple phones to an ATA 
Linear Mode
