Simplest method, ignoring cordless analog.
Set up multicall key appearances on your 3 4025's. The DN for the multicall will be a new DN and not related to the prime lines of the sets. I recommend 3 per set for a total of 9 keys. Your Dial Zero needs to be set to dial this extension. Simplest method is to create a speed call with 0 as the dialed number and XXXX as the destination digits where XXXX represents the DN of the multicall key. Setup the new DN to route to VM on Busy/no answer. The key will only be considered busy if all 9 keys are in use at the same time. The mailbox number will match the new DN (not zero). You can create Message Lamp Indicators for the new DN on an additional key on each phone to deliver message waiting for the dial zero calls. Pressing the flashing key will access the mailbox directly.
If the cordless still needs to be in the mix, you can use the cordless ext as the multicall key and proceed as above. calls directly to the cordless will integrate with VM properly and simultaneously ring the 9 keys as well.
Good luck.
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