Try this:
'snmp-server enable traps snmp linkup linkdown'
If you only want to recieve link-status traps from a single interface (serial), then you must add:
'no snmp trap link-status'
in interface config mode for every interface you don't want to recieve trap
For more information, check following:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801a80a2.html#wp1030812
HTH