Hello Rita,
>> Where I'm not clear is how to ensure VoIP server traffic always goes out the T1, and never the DSL
you want to go beyond destination based routing so you need policy based routing
see
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009481d.shtml
it has to be configured on the C2911 applied inbound on interface towards ASA per your network diagram
>> nd how to make two "static routes" (one from a T1 IP and one from a DSL IP to both go to the inside IP of the Mail server)
this means two NAT static entries, you can even define a port based TCP port specific ip nat static entry
see
The following example shows how to configure route map R1 to allow outside-to-inside translation for static NAT:
Edit:
after night thought: if you have deployed VOIP with distinct IP subnets used on both sites you can avoid PBR having specific routes advertised on the T1 link only, on the DSL link you can place less specific static routes or less specific routes.
Most specific routes are used first and this can be used here.
Hope to help
Giuseppe