I'd like to take the time to welcome you to the forums and say that I'm really sorry to hear that your phone did not disconnect the call correctly.
Normally when the other party ends the call, this will also terminate the call after a few minutes so your phone line may not have been charged after all.
To be totally honest with you, it's probably best to give our customer service team a call on 150 or 0345 454 1111 so once they pass security they can check on your calls to see if there is anything untoward.
Is this the first time that a call hasn't disconnected correctly, or was it just that the handset wasn't put down correctly (or disconnected)?
If you can confirm that the phone call was hung up correctly your end, then I'd have to ask you if you've been able to try another phone, just to see if it was the phone not releasing the call correctly.