Android : Android SIP or webRTC only for voip/audio calls?

I am fairly new in this things, my basic question is in here:

What is the best and easy way to create an audio call on web with two phones in android? Is it enough if I only use the built in Sip codes in android, or I have to use webRTC or other 3rd party libraries?

more: I have to record the calls in 3gpp or mpeg4 file format.

Thanks, Leslie