Limit maximum of active calls per phone

[elastalink]

Klaus, thanks, but this is not the answer to my question.
I wanted to know how we can configure phone to accept only up to 2 (or 3, etc.) calls on the same account.

With other vendor (Cisco), number of Line Keys mapping was defining the maximum calls per account + 1 additional call if call waiting was enabled. For example, f we have 2 Line Keys mapped to one account, 1st call was received on Line Key 1, 2nd call on Line Key 2, and if call waiting was enabled, 3rd call was received on the last Line Key (2). 4th call was rejected by the phone.