I guess standard PDA's or PDA/GSM-phones were used
they probably had a built-in Bluetooth or IR function
wich would have the advantage of not having any suspicious
unknown devices on them when checked.
In the rock was probably another PDA connected to a large battery
a simple circuit or software could wake up that PDA as soon as it
sensed Bluetooth-data..that would save on battery-power
Offcourse all data would be encrypted using lowcost and widely
available encryption-software for PDA's
A perfect example of a modern wireless "Dead Letter Drop"
They rock was mal-functioning...maybe caused by the extreme cold
they were trying to bring it in for repair.
Same could have been done over the Russian GSM-network
but they probably did'nt trust that that to much : ) :)