asterisk users - Jan 2011 - No RTP Engine problem in 1.8.2

hi guys,
i have a problem with 1.8 branch no matter which release of 1.8 i'm
using. i can't make any sip calls, this is the error message i get on
each call:

[Jan 18 19:02:15] ERROR[1698] rtp_engine.c: No RTP engine was found.
Do you have one loaded?
[Jan 18 19:02:15] ERROR[1698] chan_sip.c: Got SDP but have no RTP
session allocated.

i'm sure that the rtp engine is loaded this is the messages i get when
loading rtp engine:
> module load res_rtp_asterisk.so
Loaded res_rtp_asterisk.so
  == Registered RTP engine 'asterisk'
  == Parsing '/etc/asterisk/rtp.conf':   == Found
  == RTP Allocating from port range 1650 -> 4650
 Loaded res_rtp_asterisk.so => (Asterisk RTP Stack)

any advice to get rid of this problem?
thanks all
paradise