[Serusers] Nathelper natping-interval malfunction.

I am testing Nathelper module. It works well delivering SIP messages. However, natping-interval function seems malfunction. The NAT scenario is simulated by a microsoft ICS. There is a Cisco ATA behind NAT with internal address 192.168.0.2 and it's mapped to external address 192.17.1.22. Ser is listening on external address 192.17.1.173. I added "modparam("nathelper", "natping-interval", 5) " to the nathelper.cfg file. When ATA registered to Ser, I noticed 192.168.0.2:5056(UDP) is mapped to 192.17.1.22:1025(UDP). Register is successful and call could be established between internal ATA and external ATA(one way audio). It's expected that Ser should send a UDP packet to 192.17.1.22:1025(UDP) every 5 seconds. However, the UDP packet is actually sent to 192.17.1.22:260(UDP). I wonder what's the problem? ATA-------------------->NAT------------------------>Ser From:192.168.0.2:5060 From:192.17.1.22:1025 To: 192.17.1.173:5060 To: 192.17.1.173:5060 ATA NAT<-------------------------Ser From: 192.17.1.173:5060 To: 192.17.1.22:260 ATA NAT-------------------------->Ser Destination Unreachable