All the landline phones I've owned had a continuous tone which starts after about 30 seconds and gets to an ear piercing screech after a minute. This is the phone I have at the moment and that certainly lets me know.
http://www.cordless-phones.uk.com/img/large/009246.jpg
I think you need to get an amplified corded telephone, which will soon give an audible voice message or sound signal to indicate the receiver is 'off the hook'. gangesboy's link gives very good advice.
Incidentally, don't be bothered about the removed answers. No doubt they were stupid comments and perhaps the posters have also been removed or suspended for a while.