Limit maximum of active calls per phone

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This behavior is the same across several tested phone models: T40, T29G, T46, etc.

If we setup 1 account with 2 Line keys mapped to this Line 1 (account 1), we can receive or make the call by selecting key 1 or 2 (works as expected)

But even if we have 2 calls (1 active and 2nd on hold), phone will receive 3rd, 45th, etc. call. How we can set the limit of active calls per account?

We also tried setting up "call_waiting.enable = 0", but then only 1 call is accepted, even if we have 2 Line Keys mapped to the same account.