Do you really need to use Ringing Notification (Bells) for message waiting, or can you use MSG Waiting Lights?
Remember there is a limit on ASU and Per Nodes on how many phones can ring or have msg waiting per unit, see below from the Engineering Guidlines Handbook:
Maximum Ringing and Wake-up Calls
Maximum ringing and wake-up calls per system are dependent on the installed hardware.In all
systems, presence of any active message waiting lamps will reduce the maximum number of
ringing circuits by 25%,
SX-200 AX Controller
There is a hardware maximum of 12 simultaneously ringing ONS ports per 24 port ONSP card.
The AX also has a system-wide ringing limit of 96 ONS ports, lower than the limit of 12 ports
per card.
The same limit applies to wake-up calls on ONS ports. Maximum simultaneous wake-up calls
is 12 per 24-port ONSP card and a maximum system capacity of 96 wake-up calls.
SX-200 MX Controller with Peripheral Nodes
There is a hardware maximum of 20 simultaneously ringing ONS ports per Peripheral Node.
The same limit applies to wake-up calls on ONS ports. The maximum simultaneous wake-up
calls is 20 per Peripheral Node.
SX-200 ICP with ASU II Bays
There is a hardware maximum of 12 simultaneously ringing ONS ports per 24 port ONSP card.
Maximum ASU II capacity is 24 (12 ringing ports x 2 ONSP cards).
The same limit applies to wake-up calls on ONS ports. The maximum simultaneous wake-up
calls is 24.
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Remember that Message Waiting - Bell is NOT supported on ASU ONS ports. There is also a limitation on how many ONS stations can have msg waiting notification lights and bells per Peripheral Bay, but I cannot remember what it is off the top of my head and I can't find it quickly (something tells me it is either 20 or 31 per peripheral node though, but I am not sure)