Question: 4 year-old-son speaks while using loudspeaker option in phone and in video call.But If I give mobile phone in his hand he just keeps it in his ear for just 5 to 10 seconds and gives back to me.If I ask what they are speaking he tells me what they speaking to him but he is not replying to them.If I hold the phone in his ear he just carries on his work like playing.He is not interested in talking on phones he pushes the phone or if I give it to him he puts it on bed in my absence in that room or gives back to me.If I force to talk he just repeats what I am telling in low volume.How to make him answering others through phone?

Answer: 
With loudspeakers on, talking on the phone is just normal conversation, with the exception that you don't see the person you're talking to, comparable to talking to someone in another room.

Without loudspeakers, talking into a device and hearing a voice in another end of it is very superficial. That's something you have to get used to.

Getting silent in this situation is no different from how most adults stiffen up and act differently when they are aware somebody is filming them.

Speaking on a phone, however, is so prevalent that the kid quite certainly will get used to it and the behaviour will seem natural. I predict this'll pass, and until it does, I see nothing strange in it. I wouldn't take the fact that some other kids may not have such barriers as a concern.
As @dxh pointed out, it needs some getting used to. Some things that may help:

1. Video call, if not, speaker phone.
2. A parent providing the conversation prompts. A hypothetical from my house:  

Parent: Lets talk to grandpa about this new toy...  

Kid: hmmmm....  

Parent: How do you play with it...  

Kid: hhmmmm....runs away 
Parent: (chase the kid...) How tall did you stack up these blocks?  

Kid: It went higher than the table... and then the car flew over it ...  

Grandparent: Was it taller than you....  

Kid: Yeah! And the car zoomed like this ... (runs away...)

This needs a lot of getting used to. They need to be in the habit of talking to that person.

Also, when then talk it may not last more than a minute or so. Kids have so many things to do!

It also depends on the other person. Its easier if the grandparent talks about things that are of interest to the kid. Most of the time open ended questions - what did you do today kinds - will not work.

Even if the kid talks one day, they might not talk at all the next day. The emphasis then has to be on being polite - Hi, How are you, I am fine, Bye - level of conversation. Since the kid is only 4, the conversation can be limited to Hi - Bye.

Comparing to other kids is not useful at all. Some kids talk a lot. Some kids do not talk much. Some kids talk in different situations. If there are any developmental converns please bring it up with the child's pediatrician.