fyi, i have been battling this for a long time
https://issues.freepbx.org/browse/FREEPBX-14468
https://issues.asterisk.org/jira/browse/ASTERISK-27148
although this started for me on FBX13, it continued through v14. to be honest, the problem stopped about 2 months ago after some updates, i never got an explanation and had a hard time convincing anyone that this is a real problem…
good luck, and make sure you update.