Because I have to buy a full account for the second phone. If you already have a 2-phone family plan, a 3rd phone is $120/year. Otherwise you can get a prepaid phone for about $100/year but you pay for minutes.

However, we're not nearly there. If I call from the 2nd phone, the caller-id is that of the 2nd phone, not my master number. And for me to ring both phones, I have to have callers call a _third_ number which can do forked calling. This is not hard, but it's yet another number, not provided as the caller-id, and has minor costs (1-2 cents/minute) on all calls.

Finally, for me to do this, all phones must have voice mail turned off, because a phone that is off/out of range goes directly to voice mail which looks to the outside world like somebody answering the phone.

These are minor issues for the cell company itself to fix. Clearly their forker would understand phones that were off, and they can change the caller-id simply and easily.

Finally, I think they should sell this for less than $100/year, in fact I think it's in their interest to give it away, or worst case to sell it by the minute -- 10 cents/minute is a perfectly profitable rate on their home systems. I'm not expecting roaming from the secondary phones though that's not so hard to do either.

Doing this internally is easy, and many PBXs can do it internally today, correctly handling voice mail, caller ID and the like. It's really just an accounting decision. And trust me, there are customers who would like it a lot.