SIP_XMIT Warning Messages

In one of the Freepbx/Asterisk systems we maintain, we are seeing a lot of the following message:

WARNING[2996] chan_sip.c: sip_xmit of 0xaea3fc08 (len 490) to returned -2: Interrupted system call

Any ideas as to what is the cause and resolution?

We are using Freepbx 2.11 with Asterisk 1.8.11