This is expected behaviour, the exact same thing happens when you call someone from your landline. This is why when you do call someone and they answer the phone and the call isn't for them, but someone else who happens to be in another room with a phone, they can put the phone down on you and then get the other person to pick the phone up.
This does however mean if the person calling you doesn't put the phone down it can tie up your line, a few years back the amount of time before the line finally clears was reduced down, you can however clear the line yourself by pressing the recall button and then put the phone down and wait for the system to call you back, you pick the phone up and then when you put the phone down the line should be cleared.
However with VoIP you may find the line disconnects quicker than it would from a standard call, but will still be a delay.
