I have problem with 1760 connecting to PBX NEC, my 1760 (using FXO) can not release the connection, so when somebody try to call again the line always busy.
Not knowing the specific capabilities of the NEC PBX, may I suggest you take a look at the following url. It discusses the general problem around fxo disconnect problems, and shows different ways to solve it. Hopefully one of these methods will work for you.
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