Error building libosmo-abis, ortp missing]

Hello, I'm new around here and started from scratch with a new Ubuntu server. I followed the openBSC build guide on the WIKI page http://openbsc.osmocom.org/trac/wiki/Building_OpenBSC. Building libosmocore went smoothly Building libosmo-abis ended in an error: ===== checking for ORTP... no configure: error: Package requirements (ortp >= 0.13.1) were not met: No package 'ortp' found ===== Where can I find the ortp package? And secondly, is this the right version? I saw version 0.15.0 mentioned in the mail archives aready. Thanks in advance, Peter.