Cisco ATAs and Survivable remote site telephony (SRST) router are in one VLAN and the Cisco CallManager is on a different VLAN. When the ATA is reset from the Cisco CallManager configuration page, the ATA goes to SRST mode instead of re-registering with the active Cisco CallManager Server.
It is recommended to have all the devices on the same VLAN which is the Voice VLAN that counts for the Cisco CallManager server and other Voice gateways that includes IP phones and ATAs.
In some situations where the ATA during WAN failures do not re-register with the Cisco CallManager, the Switch port where the SRST router is configured must have must have to disable the icmp redirect functionality. For disabling icmp redirect use the no ip redirects command in interface configuration mode.
Also make sure that:
the ATA has the SRST router as the default gateway.
Disable the second port on the ATAs which is not in use. Do this in order to complete this:
On the ATA web configuration:
SCCP Parameters >> SID1 = 0 (SIDx specifies whether to enable the Phone 1 and/or Phone 2 ports on the Cisco ATA to register with Cisco Call Manager.)
With this value configured, the second port does not attempt to register at all.
Refer to EPID0orSID0 and EPID1orSID1 the section of Parameters and Defaults for more information. This is recommended because in some situations, the Cisco CallManager refuses the re-registration because there is an unknown device that tries to re-register.