[Serusers] SIP UA ACK Timedout!

Hi, I have been facing a strange problem of ACK Failed. I m using SER with SJ Phone and Xlite as softphone to make calls..but after say 10-15 mins I m getting ACK Timedout! message on SJPhone..is this SIP UA bug or something to do with SER or config? regards, Madan get_hdr_field: cseq <CSeq>: <28684> <ACK> 0(8193) DEBUG: is_maxfwd_present: value = 70 0(8193) DEBUG: add_param: tag=2124396779 0(8193) end of header reached, state=29 0(8193) parse_headers: flags=256 0(8193) check_self - checking if host==us: 14==14 && [202.71.135.212] == [202.71.135.219] 0(8193) check_self: host != me 0(8193) check_self - checking if host==us: 14==14 && [202.71.135.219] == [202.71.135.219] 0(8193) ral(): Topmost route URI: 'sip:1111@202.71.135.219;ftag=2124396779;lr=on' is me 0(8193) parse_headers: flags=256 0(8193) DEBUG: get_hdr_body : content_length=0 0(8193) found end of header 0(8193) fnr(): No next Route HF found 0(8193) ral(): No next URI found 0(8193) lookup(): '' Not found in usrloc 0(8193) Warning: sl_send_reply: I won't send a reply for ACK!!