Congrats on the interview offers! As people have said before, just act naturally. Make sure you understand and are able to talk clearly about your research experience, and could answer any questions the interviewer may ask. Be able to talk about some of the things you do outside of the classroom, what your interests are, etc. Don't focus too much on trying to stand out in some new and spectacular way--it will likely end up seeming forced/not genuine, and thus hurt you more than it hurts you. Just do you, and come across as a mature, driven, articulate young adult, and you'll be fine. If it helps, I recently had a successful phone interview for a program I was later accepted into, and I didn't do anything to make myself *stand out*. Good luck! 🙂
Also, as a side note, it helps to have some notes beside you on things you want to mention while your talking. For example, for my interview, I had an brief outline of all my research experiences, and questions I wanted to ask the interviewer.