SIP load balance criteria by call no. using irule

Set load balance to round robin. Set persistence to blank Universal with the following iRule

when SIP_REQUEST {
  persist [string range [SIP::to] 1 3]
}

This will persist on the first three numeric characters of the destination number to the backend server. If however you want exactly as you specified above then..

when SIP_REQUEST {
  switch [string range [SIP::to] 1 3] {
    123 { node server1 port1 }
    456 { node server2 port2 }
  }
}