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There can be several reasons for this, but some common issues are:

1) If using POE to power the phones, the switch providing the POE power may be underpowered and once it reaches a number of phones, any activity may cause random phones to reboot or act strangely such as degraded firmware upgrades, slow performance, etc.  Many POE switches will show all kinds of power output ratings in watts, but in the real world do not obtain or meet those specifications. In some cases the POE may be only on certain ports, or provide max ratings for a couple of ports.

2) SIP ALG issues can cause some phones to lose registration and/or reboot. Only a few models of routers can support SIP ALG in a hosted environment and should normally be turned off

3) Bad internet connections or router problems can cause phone reboots while downloading firmware

4) Missing DNS server settings in a phone can cause it to reboot or restart over and over. If static it must be set, if DHCP it will normally be relayed through the router, but in some case it may not.