twe2 is a free service that allows you to receive @replies, direct messages via SMS in other countries. They do add an advertisement to the bottom of your message, but it’s a great service. You’d be paying for each text with any phone carrier. You will still have to pay to send your text messages, unless you reply through a Twitter client in which case you would pay for the data transfer.
They will need your username and password, but they swear it is safe.