You may need to wrap it in an AGI.
Like so:
Createrandnum.agi:
#!/bin/sh
RANDNUM=`echo $RANDOM$RANDOM | cut -c1-5'` echo "SET VARIABLE
asteriskrandom $RANDNUM \"\"\n"
Call it with:
Exten => s,1,AGI(createrandom)
your should then have the variable ${ASTERISKRANDOM} in your channel
snip..