[SR-Users] change ptime / RTP packet size

hello, I use kamailio 1.5.4 with rtpproxy 1.2.1 I try to change the ptime (RTP packet size) with the Nathelper module . to do it I call the function rtpproxy_offer with the option "z20" but the "a=ptime:30" in SDP is not changed any idea ? Regards Laurent Log of RTPproxy: Jul 2 09:40:49 dns2 rtpproxy[19050]: DBUG:handle_command: received command "19072_4 UZ20c0,8,101 2ab4d3ad6ff9676a 217.162.26.26 3000 112a4c5d45;1" Jul 2 09:40:49 dns2 rtpproxy[19050]: INFO:handle_command: new session 2ab4d3ad6ff9676a, tag 112a4c5d45;1 requested, type strong Jul 2 09:40:49 dns2 rtpproxy[19050]: INFO:handle_command: new session on a port 54858 created, tag 112a4c5d45;1 Jul 2 09:40:49 dns2 rtpproxy[19050]: INFO:handle_command: pre-filling caller's address with 217.162.26.26:3000 Jul 2 09:40:49 dns2 rtpproxy[19050]: INFO:handle_command: RTP packets from caller will be resized to 20 milliseconds Jul 2 09:40:49 dns2 rtpproxy[19050]: DBUG:doreply: sending reply "19072_4 54858 15.43.10.128 " Jul 2 09:41:51 dns2 rtpproxy[19050]: INFO:process_rtp: session timeout Jul 2 09:41:51 dns2 rtpproxy[19050]: INFO:remove_session: RTP stats: 0 in from callee, 0 in from caller, 0 relayed, 0 dropped Jul 2 09:41:51 dns2 rtpproxy[19050]: INFO:remove_session: RTCP stats: 0 in from callee, 0 in from caller, 0 relayed, 0 dropped Jul 2 09:41:51 dns2 rtpproxy[19050]: INFO:remove_session: session on ports 54858/0 is cleaned up