*Update*

Same problem has been reported for calls to Westport 273 exchange. Re-routing workaround has now been put in place for those numbers also until this is resolved with Bell.
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Previous Description (2011-01-28 13:32:28):

Calls to Lansdowne 659 exchange are again resulting in "dead air". Problem was resolved yesterday by Bell Canada and will be re-opened.
Meanwhile, a workaround has been implemented in the University PBX to allow successful calls to Lansdowne.