TIP OF THE DAY :
ERROR: chan_sip.c:4271 __sip_reliable_xmit: Serious Network Trouble; __sip_xmit returns error for pkt data
This error may occur on asterisk pbx when there is some issue with sending packet to remote server due to incorrect peer configuration like peer ip address etc.,.
check the trunk /peer settings on your asterisk box. If you have multiple peers try to remove one by one and check which is causing this .