I had my phone sim locked over a year un IE, needed to go away for some time, and didnt want any abroad charges went to nearest vodaphone center, asked can they unlock my sim card... They saye'd that they usully dont do it free of charge blah blah blah, then i bought 30eur 'top-up' for my wife, and asked again politely and in 3min or so my device was 'sim-free'
Just go to your carrier, ask politely, if 'free of charge' isn't an option, buy something and then they usually unlock it for you. Of course you need to be polite and nice asking for this, if you will be grumpy and rude nobody will even consider unlocking you'r sim card cuz it isn't something in the official 'price-list'....
Bye bye HTC Sensation, it was great 2.5 years with you! Hello new HTC ONE mini!