If I add a fourth line to a Partner R4.0 system, I program the system from three lines to four lines. I have all MLS 12 sets and one MLS6. On the six button phone the three lines appeared on buttons one through three and the fourth button was for VM. That fourth button is now superceded by the fourth line? I must expand the six button phone to a twelve button phone........true. Is there any way to keep only three lines and the VM button and not have the four lines appear on the six button phone?