In Series 8 Episode 1 Deep Breath clara gets a call from a very familiar voice:
CLARA: Hello? Hello?
DOCTOR 11 [OC]: It's me.
CLARA: Yes, it's you. Who's this?
DOCTOR 11 [OC]: It's me, Clara. The Doctor.
CLARA: What do you mean, the Doctor?
DOCTOR 11: I'm phoning you from Trenzalore.
CLARA: I don't
DOCTOR 11: From before I changed. I mean it's all still to happen for me. It's coming. Oh, it's a-coming. (Back then, Clara replaced the Tardis police phone back on its hook.)
DOCTOR 11: Not long now. I can feel it.
CLARA: Why? Why would you do this?
DOCTOR 11: Because I think it's going to be a whopper, and I think you might be scared. And however scared you are, Clara, the man you are with right now, the man I hope you are with, believe me, he is more scared than anything you can imagine right now and he, he needs you.
DOCTOR: So who is it?
DOCTOR 11: Is that the Doctor?
DOCTOR: Is that the Doctor?
DOCTOR 11: He sounds old. Please tell me I didn't get old. Anything but old. I was young. Oh, is he grey?
DOCTOR 11: Clara, please, hey, for me, help him. Go on. And don't be afraid. Goodbye, Clara. Miss ya.
Then this conversation happens about why his predecessor called her:
CLARA: Well what?
DOCTOR: He asked you a question. Will you help me?
CLARA: You shouldn't have been listening.
DOCTOR: I wasn't. I didn't need to. That was me talking. You can't see me, can you? You look at me, and you can't see me. Have you any idea what that's like? I'm not on the phone, I'm right here, standing in front of you. Please, just, just see me. (Clara walks forward and studies his face carefully. Then she smiles a little.)
CLARA: Thank you.
DOCTOR: For what?
Why does the 11th Doctor call Clara's future self before he regenerates?