You may also want to post your query to Doorbird as it is their device that has to support the ability of what I assume to be direct IP dialing to the phones. The GS equivalent device supports this using parallel mode (the ability to dial more than one device) with IP dialing.
Also, without knowing any of the details which Doorbird model or how the video is set - codecs, payloads, bit rates and the like, the question is rather open ended. I looked at what little documentation I could find, but I was not able to find any info that led me to believe that the device is capable of talking to more than a single SIP point. I would not take my finding as the answer as I am not familiar with it, but hopefully someone else using the same will see the post and advise.