[EDIT: Wiring diagram to connect PAP2 to RC2a and E20B doorphone can be found HERE]
As a VoIP tech, I usually leave the old PBX phones in place when we install a new VoIP system. I do this for several reasons.
First, to allow the porting to take place which can take a few days. The reasons why I do not like to just use call forwarding are explained in my LNP post but basically when call forwarding a number only the first call is forwarded then all subsequent callers get a busy signal.
Second, it allows the customer to transition to the new phones at their own pace for outgoing calls until the port process is complete.
Today I had a rather unusual request, as most people that move over to VoIP eventually do away with their old phone system and POTS phone service. This customer has an Partner phone system and VoIP service with us with Cisco phones, so each desk has a VoIP phone and an Avaya phone. This customer wanted the door phone, which was currently run thru their Partner system to at the same time ring the VoIP phones.
After some deliberation, a solution was found to accomplish this. The existing door phone was simply connected to the partner system as an extension. I tapped off of this extension wiring going towards the door phone after it exited the Partner phone system and connected the station wire to the tip and ring of an RC2A relay, then connected the N.O. and common connections of the RC2A to the existing wiring going to the door release. That solved the connection of installing my relay.
My next hurdle was the tricky part, how to ring the VoIP phones AND the phones connected to the PBX. The soloution? Connect the line side of a SPA3102 to an extension port of the Partner system, and configure the SPA3102 to hotdial an extension group on the VoIP system when it detects ringing on the extension port that it is connected to, thus whenever someone rings the door phone all of the extensions on the Partner system ring, and since the SPA3102 is connected to an extension port on the Partner system it too rings, however when the 3102 is ringing, it is forwarding the call via VoIP to the VoIP phones.
Sorry no pictures this time, it is pretty straight forward, nothing too fancy it just works. The RC2A was put in line with the door phone station wire to activate the relay with DTMF tones - if the existing relay would have used DTMF tones to operate the relay it would not have been required, I just would have had to determine the proper digits to dial to activate the relay with a DTMF decoder.
[EDIT: There are 2 things to keep in mind with this set up. First if anyone calls the extension that the 3102 is connected to, whatever VoIP phones that are in the door bell call group will ring, and the CID will show that the call is coming from the door phone. The other thing to be concerned with about this set up is if the extension that the 3102 is connected to rings when someone calls one of the CO lines that the analog PBX is connected to all of the VoIP phones that are in the door bell call group will also ring and CID will show it it coming from the doorphone. If someone answers one of these calls with the VoIP phone then the doorphone will not be able to be used while this call is in progress, so you will have to ensure that the extension that the 3102 is connected to in the analog PBX is not in any call groups on the analog system, which may be a problem if you are not familiar with programming of the analog phone system that the customer has.]