Hello, Gile
Command 9001 is for digital phone assingning multiline extension and this command defines weather the extension assinged on the multiline button will ring or not and by default this is enabled.
To check your problem first identify if what type of terminal are you assingning is it digital or analog second you must be sure if the LEN whre you assign the extension is same port that your phone is terminated on the equipment terminal and third double check your euipment cabling side be sure that the cable is not split because if it is split then cross line party is possible because you are getting another line from the split pair of equipment cabling...
assigning extension is simple for digital you must assign in 14>LEN: Fxxx, 9000>Fxxx,01:xxx, 93>xxx:xxx for anaolg you just assign on 14>LEN:xxx, and ringer wont be a problem not unless the ring tone genarator unit(RGU) of the power supply for the PIM where the extension you are assigning is deffective check if all the locals on that PIM don't have ringer then it is a power supply problem digital phone has it's own ring tone generator unit (RGU) so digital wont be affected even if the RGU of the power supply is deffective.