I left out two last sentences, Writeacher. Could you please check them?

Question

1) I have no credit on my phone. I need to top it up.
3) They are trying out a new vaccine against cancer.

Answer 1

In American English..

I have no minutes left on my phone, I need to refill.

They are experimenting (or trying out) a new vaccine to (prevent) (cure) cancer. I am not certain of your purpose intended for the vaccine.

Answer 2

1. This is fine. I think "top it up" is a British phrasing that's not used in the US, but it's correct.

3. ... a new vaccine to prevent cancer.

Nice!