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

Question

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?

Answer 1

Since the Doctor knew he was going to regenerate in this scene, and he also looked young-looking, I think we can presume the Doctor made this call in between the following two scenes in "The Time of the Doctor":

1. The scene where a very elderly-looking Eleventh Doctor suddenly realized he had another regeneration and used the regeneration energy to blast the Daleks:

DALEK: The rules of regeneration are known. You have expended all your lives.

DOCTOR: Sorry, what did you say? Did you mention the rules? Now, listen. Bit of advice. Tell me the truth if you think you know it. Lay down the law if you're feeling brave. But, Daleks, never, ever tell me the rules!

DALEK : Emergency! Emergency! The Doctor is regenerating! The Doctor is regenerating!

DOCTOR: Oh, look at this. Regeneration number thirteen. We're breaking some serious science here, boys. I tell you what, it's going to be a whopper!

DALEK: Exterminate! Exterminate the Doctor.

DOCTOR: You think you can stop me now, Daleks? If you want my life, ha, ha, come and get it!

2. The following scene at the end of the episode, where Clara entered the TARDIS and found a young-looking Eleventh Doctor waiting for her, who explained his youthful appearance as a "reset" immediately prior to regeneration:

CLARA: Doctor!

DOCTOR: Hello.

CLARA: You're young again. You're okay. You didn't even change your face.

DOCTOR: Ha! It's started. I can't stop it now. This is just the reset. A whole new regeneration cycle. Ooo.

Also, you mention that in the phone call in "Deep Breath" we see a shot of Clara replacing the hanging phone on the TARDIS, a shot originally from "The Time of the Doctor"--and this scene did indeed happen between the two scenes I mentioned above, it's at 58:35 on the US blu ray (might be a little different on blu rays from other regions since the US blu ray is slowed down to 24 frames per second, whereas the episode was broadcast at 25 frames per second, a speed US televisions aren't designed to play).

Answer 2

He knew because this was after he got his second regeneration cycle, and his 13th regeneration was in it's final phase: the change ("Oh, it's coming"). Even then, he kind of described the regeneration as a death type thing, like his previous incarnation, but not obsessing and trying to avoid it. He wanted to Clara to know that, even though he might not exactly be the man she once knew, he's still and always will be the same caring, kind, witty Doctor, that she, and many others before her knew, and grew to respect and love. So there you go.