I'm not sure that your error is specific to freetdm, check your public dialplan, in the place where you route your number to 1100, you probably have it transfer to "1100 XML $some_empty_var"
2010-11-17 02:10:16.853170 [INFO] mod_dialplan_xml.c:331 Processing <1234012480>->1100 in context 2010-11-17 02:10:16.853170 [WARNING] mod_dialplan_xml.c:361 Context not found