The current stable version of net-voip/linphone, 3.5.2, requires >=net-libs/ortp-0.20.0 and <net-libs/ortp-0.22.0. Given the two newest stable versions of ortp are 0.20.0 and 0.23.0 (there is also 0.22.0 but it is unstable, moreover this doesn't change anything; the important bit is that there is no 0.21.0 in Portage), the former gets pulled in. However, that version of linphone also depends on media-libs/mediastreamer-2.9.0 - which in turn requires >=net-libs/ortp-0.21.0, pulling in version 0.23.0. As a result, the current stable version of net-voip/linphone cannot be emerged. The easiest way of working around the problem is to use the latest, unstable version of linphone i.e. 3.6.1, which depends on >=net-libs/ortp-0.22.0.
*** This bug has been marked as a duplicate of bug 545196 ***