Dave H. has basically answered your question as to setting up dual line extensions, then cascading the calls to the next extension/button.. So, with that said, the SPA509 has 12 buttons, and can theoretically support up to 24 calls. Not something that I would suggest doing though..
Are park slots not an option? You use up system resources then instead of locking up the phones resources... Or is there a specific need to create more Dual-Line keys on the phone??
Is this for a busy reception environment? I can see any other reason that this would be needed unless you have a phone that is trying to handle quite a few calls in which case the SPA-500 series is not really the right phone, an Octo-Line phone would be best suited for that operation (7900 series phones).
Just some food for thought
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