Rhea, if I remember rightly you can get a sim card reader that does read deleted messages. I have one at home and I am sure that I reads any messages. I think that it stores up to 200 messages, whether deleted or not. Available from E bay. I am away at the moment I will have a look at the instruction book when I get home on Friday.
Having spent a considerable amount of time recently, with a specialist and the companies involved in exactly this, interception, and jamming, consider your 'phone an 'open book', to anybody that seriously wants to know what you've been up to.
ALL is possible, but probably extremely expensive.