Hi Jayesh, By default, nathelper will not send a SIP message but a 4-bytes quick probe. This is not supported by many CPE devices. In order to use the bigger/more expensive OPTIONS ping, use the following nathelper options : modparam("nathelper", "natping_interval", 30) modparam("nathelper", "ping_nated_only", 1) modparam("nathelper", "sipping_method", "OPTIONS") modparam("nathelper", "sipping_from", "sip:pinger@OPENSER_DOMAIN") Of course, replace the vbalues according to your own settings. According to the RFCs, the UACs should respond 200 OK to the options message, so this validates the NAT in both directions. I confirm that this is the case in my settings, both linksys PAP2 or audiocodes MPXXX gateways that we use respond 200OK to the OPTIONS keepalive. In that scenario, it is useless to make your CPE devices send their own keepalive messages. Note that the traffic can be very high when you're pinging the devices with OPTIONS messages, because all the keepalive messages are triggered at the same time. So be carefull to select a reasonnable natping_interval. Here, 30 seconds is really expensive in my situation, but my openser is hacked so the keepalives are spread out in time. If you need the keepalive to be spread out in time and not sent all at once, look at this message thread in devel ML : Subject: Re: [Devel] [Fwd: [Serdev] CVS:commitlog: sip_router/modules/nathelper nathelper.c nathelper.h natping.c] Date: Tue, 24 Apr 2007 19:18:06 +0300 (18:18 CEST) Also, take a look at nathelper module documentation, because you need to set the NAT flag so nathelper can ping NATed contacts only. Hope this helps, Cheers, Jerome