Webtxt app


I have an HTC Desire Z and am off on holiday next week. After speaking to TMobile today I have discovered that there is no Booster that will cover txt messages deom the Canary Islands back to the UK. I'm wondering if there is an application a bit like Whats App but will give me access to all my contacts not just the ones with said application already downloaded so I can continue to txt whilst not racking up larde bills. I was able to add the International Internet booster so though a Webtxt application may be good. Unfortunatley the TMobile site is unable to handle this at present....Would appreciate the advice. Thanks