How to get the caller phone number of incoming calls with SIP.js on the browser?

Question

I am using SIP.js in the browser using SimpleUser (SIP over WebSocket). I am connected and registered. When a call comes in details (including caller phone number) are logged into the JS Console. My onCallReceived function is called. But it has no arguments.

So how do I get the phone number of the caller?

I do not want to answer the call and I do not want to reject the call. I just want the number of the caller. The call is answered by an other device.