RTP is a protocol to send media streams over networks.
Linphone uses default port 7000 to send and receive audio streams. If you think port 7000 is used by another application, change it as you want.
This number represents the number of audio packets linphone is waiting for before starting to play them. If sometimes some audio packets are late, they have more chance for being played. Increase this parameter if you hear 'cutted voice' to improve the quality of the transmission, but it will increase the delay (you will hear the voice of the remote user a few second later). On the other hand, if you are using a perfect network, and if you have good audio drivers, you can set this parameters down to three packets, and so you will have a short delay.