The base stations M300, M700, M900 support DHCP Option 120 defined by RFC 3361.
DHCP Option 120, if configured on your DHCP Server, will provide the base with the optioned SIP Server information. The DHCP option will override the configured SIP server.
You can disable this behaviour disabling the DHCP Plug-n-play setting (you can find it under Network -> DHCP).
This setting can also provisioned via the dhcp_option_pnp setting tag.