In some instances a phone will not boot up properly or loose registration. the following recommendations are to assist in getting your VoIPZone services restored at your home office:
Open the following ports in the firewall:
SIP UDP 5060 through 5085
RTP UDP 10000 Through 20000
DNS TCP/UDP 53
SNTP TCP/UDP 123
Also look through your router and find either SIP ALG(Application Layer Gateway) or SIP FixUp and make sure they are disabled.