@compileandpush I've used it in a relatively small scale, but given the low load, I think you can probably have hundreds of simultaneous calls with just one instance of Janus and one instance of your Sip server. As long as you don't transcode inside Janus it will perform extremely well
Janus has a plugin (just enable in the config) for sip. It will natively bridge incoming and outgoing calls between the browser and the sip server. You then need to write all the front end code for the phone functionally, but the heavy lifting is done on the back end