Hi Suchi,
This is covered in the design guide, plz check the text highlighted in bold
Egress Gateway Selection for Roaming Devices
Typically the desired egress gateway selection behavior of roaming devices is to use gateways local to the visited site. The recommended way to implement egress gateway selection that is specific to the calling device is to use PSTN route patterns pointing to route lists that use Standard Local Route Group. Using Standard Local Route Group in a route list effectively means that Standard Local Route Group, when routing an actual call, will be replaced with the Local Route Group configured in the device pool of the calling endpoint. This schema ensures that site-unspecific route patterns and route lists are used; site-specific egress gateway selection completely relies on device pool-level Local Route Group configuration.
For roaming devices (whether roaming inside or between device mobility groups), the device mobility feature always ensures that the Local Route Group of the roaming device pool is used as Standard Local Route Group. This guarantees that, with Local Route Group egress gateway selection, a visited site-specific route group (and thus gateways local to the visited site) will typically be used. This behavior ensures that, for example, emergency calls routed via route patterns that use a Standard Local Route Group route list will always use egress gateways local to the visited site.
Local Route Group egress gateway selection can be used with all dial plan approaches explained in the chapter on Dial Plan.
If certain calls from roaming endpoints need to be routed through gateways local to the home site of the roaming phone, then routing for these calls has to be implemented through route patterns pointing to route lists that use fixed site-specific route groups instead of Standard Local Group.
In a line/device dial plan approach, these route patterns would be addressed by the device CSS configured on the endpoint. When roaming but not leaving the device mobility group, the calling endpoint's device CSS is replaced by the Device Mobility CSS configured on the roaming device pool. If fixed egress gateway selection is required for some calls and the route patterns for those calls are addressed by the device CSS, you have to make sure that roaming devices always roam across device mobility groups. This will guarantee that roaming endpoints always use the device CSS configure on the endpoint.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/9x/uc9x/mobilapp.html#wp1236797
HTH
Manish
