How did the 11th Doctor know that he was going to survive?

From the transcript of Doctor Who (2005) S08E01:

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?

[Trenzalore / City street]

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.

In the next scene (which was from episode The Time of The Doctor), it was shown that Clara was putting back the phone inside TARDIS. At that point, how did The Doctor come to know that he was going to regenerate but not die? Didn't he know that he already used all of his 12 regenerations and he was going to really die?