1. I am guessing "mobile" is British English. That sounds like something they'd say. In the States, we wouldn't use this term. We'd usually just say "phone". Anyway.... I think you should say "is running a custom version" if you are talking about software. If hardware, you'd just say "is a custom phone". 2. You'd say "a custom phone" or a "customized version of". 3. All of those seem fine. The last seems correct but I wouldn't use it. 4. "through customs"