You can view SIP device status from asterisk command line.
# asterisk -vvvr
> sip show peersor if pjsip
> pjsip show endpoints
If using Freepbx GUI you can also go to
Reports > Asterisk info > Peers
To determine if it is a problem with the SIP device you can try use a free softphone such as Xlite, Zoiper, PhonerLite etc. to see if the behavior is duplicated.
For NAT routers/firewalls check if they have a SIP ALG feature. If enabled try disable and if disabled try enable. Also any settings for gaming or other port forwarding should be disabled to see if that makes a difference.