One presumes the system could work as follows:

a) Blank clips are manufactured by Mykotronx and VLSI.  The number
produced is carefully audited and they are shipped to the first
escrow house.

It programs the chips with its half the key, and prints out a paper slip
with the key half and non-secret chip serial number.   The reams of paper
are filed in locked boxes in the vault, a fuse is burnt in the chip so
that the key is now unreadable.

The chip then goes to the next escrow house, where the same thing is
done.  This continues through N escrow houses, perhaps, could be more than
2.

The last one provides the chip to the cellular phone maker.

And yes, this has to be a public key system or it would be almost
impossible to handle.  It might not be RSA, but that does not mean
that PKP doesn't get paid.  Until 1997, PKP has the patent on the
general concept of public key encryption, as well as the particular
implementation known as RSA.