Sounds like you are not making enough fuss to customer services!
I phoned orange up a couple of months ago as I was getting the same stuff.
First they said they were in stock and I could have a really decent contract with free phone!!!, but... literally phoned about 20 mins later to make an order and it had changed to 300 mins less and £50 for the phone! and the best part was they said stock would be another 2 weeks to a month and I can't go on the waiting list as it's full!
Pissed off, I decided to go to high street stores to get a more expensive contract and alas, no-one had any and if they did I was told that the tariff I was on would mean I would have to pay £75 for the phone!!!
So I got home, got myself in a strop and phoned orange to complain, funny enough they added me to the list (before it was full), they said when the phone was in stock they would text me and as soon as the text came through I needed to call them straight away as it was first come first serve after the text was received.
Patiently waiting (not) I decided to call the next day to find out whether it had come in, they said yes and were sending the texts out to everyone now, with 6000 odd people on the list I didn't fancy my chances!
Somehow while trying to get them to haggle on the contract (cancellations dept) which they did in the end - gave me a great contract, god opened up the heavens and I got my text whilst on the phone, the lady sounded like she didn't believe but I gave her the code and she said congrats and that I could reserve a black one!!
The next day there is my shiny new Desire on my doorstep!!!
The End... lol