"Hello" and "Goodbye"

I've noticed something.  Saying "hello" or some other such greeting at the beginning of a professional phone conversation is still the norm.  However, I have also noticed that saying "goodbye" at the end of the same  conversation seems to be something of the past.  At my work, I answer the phone an average of 25 times a day.  I have become aware of a very strange trend over the past 6 months or so.  It started out with phone conversations with folks working at other store locations.  At the end of the conversation, I would say "goodbye", "Ok thanks", "Bye",  "bye-bye" or something to signal that my end of the conversation was complete, only to discover that the other person had already hung up!  Seriously, I chalked it up to the idea that they were working in a larger volume store in the "big city" and that they were, for lack of a better word,....rude.  Nope.  Now approximately six months after I initially became aware of this trend, I find myself being hung up on almost 1/2 of the time when the conversation has ended.  Not just by other employees in other stores and not just by those living in the "big city".  This is a trend that I personally do not like.  It leaves me hanging....guessing really.  Did the cell phone loose reception?  I get left with so many questions!

Here is an example of what I am talking about.  My manager was on the phone today when another call came in for him on the other line.  Here is the gist of the conversation I had with the person on the other end:
Me: "Thank you for calling Sherwin-Williams, Yvette speaking"
Customer: "Hey, this is _________, is Mike there?"
Me: "Oh hi, _______, Mike is here but he is currently on the other line.  Is there something I can help you with?"
Customer: "No, I need to talk with Mike."
Me: "Can I take a message and have him call you back or would you like to hold?"
Customer:" I'll hold."
Me: "Ok, thanks." (place customer on hold and after about a minute when I see that Mike is still on the other line, I get back on the phone...as to not waste the customer's cell minutes and say) "_________, Mike is still on the other line.  May I have him call you back?"
Customer: "Sure.  Do you need my cell number?"
Me:" I am not sure he has it, so let me jot it down real quick." (Customer gives me his cell number and I repeat it back.)  "Ok so I will have Mike call you as soon as he is off the other line."
Customer: "Ok, thanks." (phone clicks as he hangs up

Now it might seem strange to some, but I find it offensive to hang up without saying goodbye!  Maybe I am just one of those people who need closure, but it seems so rude to me to just hang up.  Is there anyone else with me on this one?  Just hoping I am not alone in my thinking.  Oh and before I close...."Have a great day and goodbye."