Such a vague question is hard to answer without knowing where you're going wrong.
You need to try to think out answers to possible questions in advance, so that you'll sound both confident and keen. For example, if you're asked "Why do you want to come and work for Stuffit Ltd?", hesitating for quite a while before mumbling something along the lines of "Well, I need a job, don't I?" probably won't get you very far! It would be much better to say something along the lines of "I think that I'd be good at the job because I enjoy working as part of a team".
Also, obviously(?), it's best to avoid 'street language' and to stick to 'business language'. Don't start a sentence with 'So' (unless your statement really does follow logically from the previous one) and avoid saying 'like' and 'you know' in the middle of sentences.
Remember the golden rule (that was repeatedly drummed into us at school) is that one should never respond to a question with a simple 'Yes' or 'No', as it's regarded as 'ignorant'. So, if you're asked "Do you think you'd like working here?", don't just say "Yes". Amplify your response along the lines of "Yes, definitely. For one thing, I like the idea of working permanent late shifts, as I'm definitely a night owl. I also enjoy working where there's mix of routine tasks and more challenging ones".
Lastly, if you've got a strong regional accent, try to tone it down a bit on the phone. For example, if you're from Glasgow, you've every right to be proud of your accent but if you sound like Rab C Nesbitt on the phone, with a potential employer struggling to understand what you're saying, you're probably not going to make a very good impression.