“It's Tina.”
It's Tina. identifies the speaker on the phone. This is Tina. is another standard form.
A2-59 / CEFR A2 / EPISODE 59
Asking about a train delay
SceneTina and Frank are at the train station. Bad weather has caused a delay, and Tina has a group presentation at school.

WORDS & CONTEXT
It's Tina. identifies the speaker on the phone. This is Tina. is another standard form.
Without me means in my absence. Expecting to miss the start of the presentation, Tina tells Ken to have the group begin before she arrives.
Not the end of the world. literally denies a catastrophe but means the problem is not that serious. Frank uses it to calm Tina about possibly being late.
I've gotta go is a set phrase for ending a conversation: "I have to go now." See the note in A2-24 for gotta. Before hanging up, Tina adds a farewell: See you in fifteen minutes!
Forever means for all time. Ken picks up Tina's earlier Goodbye forever. and checks: "So it's not goodbye forever?" Tina, thinking she would miss the presentation, said a dramatic farewell, then switched to "See you in fifteen minutes!" the moment she learned she could take a taxi. Ken's comeback on that sudden change is the punchline.