SwiftUI + PhoneNumberKit implementation

SignInView.swift

import SwiftUI
import UIKit
import PhoneNumberKit

struct PhoneNumberTextFieldView: UIViewRepresentable {
    @Binding var phoneNumber: String
    private let textField = PhoneNumberTextField()
 
    func makeUIView(context: Context) -> PhoneNumberTextField {
        textField.withExamplePlaceholder = true
        //textField.font = UIFont(name: GlobalConstant.paragraphFont.rawValue, size: 17)
        textField.withFlag = true
        textField.withPrefix = true
        // textField.placeholder = "Enter phone number"
        textField.becomeFirstResponder()
        return textField
    }

    func getCurrentText() {
        self.phoneNumber = textField.text!
    }
    
    func updateUIView(_ view: PhoneNumberTextField, context: Context) {
  
    }
    
}

struct SignUpSignInView: View {
    @State private var phoneNumber = String()
    @State private var validationError = false
    @State private var errorDesc = Text("")
    @State private var phoneField: PhoneNumberTextFieldView?
    let phoneNumberKit = PhoneNumberKit()

    var body: some View {
        ZStack {
            VStack(alignment: .leading) {
                Text("Sign in with your phone number")
                
                self.phoneField
                    .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 60)
                    .keyboardType(.phonePad)
                
                Button(action: {
                    do {
                        self.phoneField?.getCurrentText()
                        print("phone is: \(self.phoneNumber)")
                        let validatedPhoneNumber = try self.phoneNumberKit.parse(self.phoneNumber)
                        print("Validated Number: \(validatedPhoneNumber)")
                        // Integrate with your login/registration system here...
                    }
                    catch {
                        self.validationError = true
                        self.errorDesc = Text("Please enter a valid phone number")
                    }

                }) {
                    Text("Sign in")
                }
                .padding(15)
                Spacer()
            }
            .padding()
            .onAppear {
                self.phoneField = PhoneNumberTextFieldView(phoneNumber: self.$phoneNumber)
            }
            .alert(isPresented: self.$validationError) {
                Alert(title: Text(""), message: self.errorDesc, dismissButton: .default(Text("OK")))
            }
        }
    }
}

Use more canonical and idiomatic Swift code if you are targeting iOS 13.0+

import SwiftUI
import PhoneNumberKit

struct PhoneNumberField: UIViewRepresentable {
    @Binding var text: String
 
    func makeUIView(context: Context) -> PhoneNumberTextField {
        let textField = PhoneNumberTextField()
        
        textField.withPrefix = true
        textField.withFlag = true
        textField.withExamplePlaceholder = true
        textField.withDefaultPickerUI = true
        
        textField.delegate = context.coordinator
        textField.becomeFirstResponder()
        return textField
    }
    
    func updateUIView(_ uiView: PhoneNumberTextField, context: Context) {
        uiView.text = text
        
        guard let phoneNumber = uiView.phoneNumber else {
            return
        }
        
        if phoneNumber.countryCode == 82, phoneNumber.type == .mobile, String(phoneNumber.nationalNumber).count != 10 {
            // Obsolete Korean mobile phone number
            // e.g. 011-123-4567
            return
        }
        
        DispatchQueue.main.async {
            uiView.resignFirstResponder()
        }
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    class Coordinator: NSObject, UITextFieldDelegate {
        var parent: PhoneNumberField

        init(_ parent: PhoneNumberField) {
            self.parent = parent
        }
        
        func textFieldDidChangeSelection(_ textField: UITextField) {
            if let text = textField.text {
                parent.text = text
            }
        }
    }
}