Here's the first part...Sounds very good! Very natural...
Dinnah: Hello, May i talk to Robert?
Dinnah: Hello! May I talk to Robert?
John:Who is speaking.(or) Who is there?
John: Who is this? of Who's this?
Dinah:This is Dinna Speaking, Robert's friend.
Dinah: This is Dinah, Robert's friend. (Or, This is Dinah, I'm a friend of Robert's)
John:Sorry Dinnah Robert is not home at the moment.
John. Oh. Sorry Dinah, Robert's not home (or Not here at the moment.)
Dinnah:Oh, it is ok. I will call him later.
Dinah: Oh, okay, I'll call him later. (We rarely say "will"... almost alway use the contraction 'll)
John:Actually,There are some guests coming from London, he has gone to receive them at the airport.
John: Actually, he has some friends (some folks, some people) coming IN from London, and he went to GET THEM FROM (or MEET THEM AT, or PICK THEM UP AT) the airport.
Well, If you have any message for him, let me know i will give him your message when he will be back home.
If you want, I can take a message for him for when he gets back.
Dinna:No. But plz do tell him about my phone call.
Dinah: No, that's okay. But please do tell him about my call, OR Please let him know I called.
Johh: Yeah, sure. OR Yeah, okay, I will OR No problem, I will.
Dinah:Thanks........Oh, i did not ask ur name.
Dinah: Thanks... Oh, I didn't ask your name OR Oh, I'm sorry, what was your name?
John:So what? You can ask my name now...haha. By the way i am john, Robert's brother.
John: So what? You can ask my name now... ha ha.. (Very good writing! Cute joke...)
By the way I am John, Robert's brother.
I'm John, Robert's brother.
Dinah:Oh, nice to talk to you, John.
John: the same here.
John: Same here.
Dinah:I hope we will see soon.
Dinah: I hope to see you soon.
John: Sure. And i am sure that u will be as happy to see me as happy u are to talk to me.
John: Sure. And I'm sure you'll be as happy to see me as you are happy to talk to me.
Dinah:Haha..........ok let's see. Bye Jojn.
Dinah: Ha ha... we'll see. Okay, bye John.
John: Okay, bye... OR Okay, see you later.. OR Okay, see ya later.. OR Cya later.