I have decided that contrary to Avaya techs, you can keep adding true Partner Doorphones because they operate like an analog telephone. You won't get the "ding-dong" tone on doorphone 3+ but you can program the calling location so it will display its calling location with a standard CO ring tone.
We now use two regular Partner Doorphones, two Door Controller Doorphones (so we can directly release doors through phone system), and added a Viking E-30 using an open CO port. We hope to add two more doorphones using the Viking E-30's. By using the CO ports you can assign the "door-lines" to whichever telephones should answer/respond to the particular door(s). This works till you run out of CO ports.
Welcome your thoughts and comments.
I believe you can run two or more Viking door units with proper Viking controller. Training staff might get tricky with more doorphone types! A Viking 3000 door unit will allow dialing directly to a specific extension so we might try one but they're pricey, too.
If all else fails I look to Viking for solutions but, it requires some thought and nobody gives their gear away!