I have a similar setup that I use, essentially setup a queue that phones can dial into and it will play.
I wanted to see about exploring further an option to just have it use multicast. I believe @dicko mentioned an idea in a thread, and possibly during the OpenLounge once. I wanted to have a way to have the phones play a multicast stream, that way, you could still receive a call or other multicast streams on top of this. Making it very similar to how the Panasonic would have worked. My setup, we replaced a Toshiba system.
My setup is primarily with Sangoma phones and Yealink phones. I know for sure with the Sangoma phones, I got it setup that while on a call, they would put the call on hold (ie, you called into the BGM queue) when receiving a multicast page. So, similar to how it worked previously, but they have to resume the call to hear the music.
If someone else has an idea on how to setup Sangoma or Yealink phones to play multicast streams, I would definitely be willing to work and test it out with our setup. For now, we just have a speed dial that staff can hit to put them into the BGM queue, and listen to music that way.