Hello, we have already created just such an application which is currently being beta tested, but seems to work well. You can chat to your live customers via email and hence any phone in the world which has email capabilities (Made specifically for blackberries but will work with most phones or email clients).
We are the developers of this technology called the Abhiware Chat App Pro.
If you are interested in getting a license when we release it officially you may email us and you can see it in action at cheapest-website dot com where we use it to help customers in the purchase of a website.
Our website is abhiware dot com so you can go there and email us as we are still in beta tests with the technology getting it ready for production and finalization.