Switch Universal App to iPhone Only App

Is it possible to change the universal app to iphone only once it is uploaded to app store?

UIRequiredDeviceCapabilities would help you.

Put telephony value like this in info.plist (IPhone specific...)

Of course you need submit your app again.

UIRequiredDeviceCapabilities

UIRequiredDeviceCapabilities
UIRequiredDeviceCapabilities (Array or Dictionary - iOS) lets iTunes and the App Store know which device-related features an app requires in order to run. iTunes and the mobile App Store use this list to prevent customers from installing apps on a device that does not support the listed capabilities.

Sample Image

Change compatibility of submitted app from universal to iPhone only

Yes, that should do it according to this other stack overflow post:

How to Restrict the iOS app only for iPhone excluding iPad?

Essentially there's no way to restrict apps by device unless the device doesn't have a feature you need, in this case, the ability to make calls (telephony).

Converting Universal app to iPhone only app

if u're using xcode 4.2

navigate to project navigator
select your project
selct target > your project's name > summary
change the devices from "universal" to "iphone"

Update ios Appstore app to allow only iPhone

It is not possible with the original Bundle ID.

Citing from the doc below:

https://developer.apple.com/library/content/qa/qa1623/_index.html

Bundles must continue to support any devices previously supported.

The only option for you is to create a new bundle ID and submit your app with it.

Removing your app from the store, and uploading the update with a
different bundle ID, will allow you to narrow the range of devices
your update supports. However the update will be listed on the store
as a separate app.

iOS: Universal App - Release as iPhone only

It's not possible to only release the iPhone version via iTunes Connect, but you can set the Target Device to iPhone-only for the first version and it'll only be released for iPhone/iPod touch.