The recorder will record the RTP .i.e the audio of the Voip call
For example ,If you a make a phone call to another phone and there is no answer on the other end,the phone just rings or goes straight to Voicemail,the recorder will record the ringing of the phones and/or the voicemail
The same applies for softphones but sometimes on softphone you only hear one ring then silence then another ring from the destination telephone.
On this instance only those rings will be recorded.
The recorder does not record Comfort noise.