Could you answer the question from engenius: "Did you assign 2 line keys in Nortel settings?"
To make a page call, you need a free line appearance key. If you have only one line appearance key and you put the call on this key on hold, you couldn't make a second call. Go to your extension's configuration page, select Nortel Properties and look at the Number Of Lines value. If it is set to 1, you can have one and only one call on your phone...