It didn't then. In the normal course of events, only one phone was on
at a time. But whenever I had more than one on at a time, all would
ring, and I could answer the call from any of them.

The ESN is registered with Sprint.  You *can* program multiple phones
with the same number but only one will work.  (I've been through this
a lot recently while trying to switch to a phone that works reliably
after my old one became flakey.)

You can certainly have calls to a single non-SprintPCS phone number
ring multiple Sprint phones though.  I often have calls to our home
phone number ring my phone and my wife's (in addition to our home
phone).  A little VoIP magic is all it takes.

'course if SprintPCS would just offer VoIP service for their mobile
phones stuff like this would become really clean and easy to do.

--kyler

In the situation I had with the three car phones, the phones were NOT
cloned. Each had the same phone number programmed in, and retained
their unique ESN. The Cellular One system handled it, and any
programming changes occurred at their side, not in the phone.

The service that the OP had (and I had and hundred's of others had) was not
cloning. It was all done in the switch and the phone itself actually had a
different number. The "call forwarding/ringing of several numbers" feature
of the switch was used.

It did actually have service.  I called voice mail with the new
phone, and then with the old phone, and they both worked.

But then later that night, the old phone had lost service,
and it can't make calls now.

So obviously it was a transient effect: the network update
to recognize the new phone ID was immediate, and the one to
disrecognize the old one took more time.  Too bad I didn't
try calling myself while I had the chance; now it's too late.