After 1st telephonic technical round, I got the call to come to BoA office in Hyderabad for technical discussion of 2nd round.
That interviewer was good & patient enough to listen the ideas/answers. He even helped with hints when getting stuck.
Below are the coding questions asked apart from some theoretical questions, though I was not able to write the complete code except to reverse the String in simple way. But below I have mentioned one more way to reverse the String plus I have added the code for other 2 problems -


import java.lang.reflect.Field;
import java.util.Collection;
import java.util.Iterator;
import java.util.Queue;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;


public class BoA1 {
 
public static void main(String[] args) throws NoSuchFieldException, SecurityException, IllegalArgumentException,      
 IllegalAccessException {

  String str = "123456789";
  System.out.println("Converted to integer : " + integerAns(str));
  reverseString(str);
  System.out.println("Reversed the string : " + str);
  BlockingQueue<String> bq = new ArrayBlockingQueue<>(20);
  BlockingQueue1<String> bq1 = new BlockingQueue1<>(20, 1000, 100);
  Producer prod = new Producer(bq1, 30);
  Consumer consumer = new Consumer(bq1);
  Thread t1 = new Thread(prod);
  Thread t2 = new Thread(consumer);
  t1.start();
  t2.start();
 }
 
// Question 1: Parse the string containing integers only, without using any Java API.

 private static int integerAns(String str) {
  int len = str.length();
  int ans = 0;
  for(int i = 0 ; i < len; i++) {
   char ch = str.charAt(i);
   int n = ch-48;
   int k = len-i;
   while(k > 1) {
    n = n*10;
    k--;
   }
   ans+=n;
  }
  return ans;

 }

 // Question 2: Reverse the given String. Below way can be used to reverse the String.
 // It reverses the provided string, if dont want to reverse the original String then skip
 // Reflection part.
 private static void reverseString(String str) {
  try {
   Field field = String.class.getDeclaredField("value");
   field.setAccessible(true);
   char[] chars = (char[])field.get(str);
   int len = str.length();
   int last = len-1;
   int i = 0;
   while(last > i) {
    char temp = chars[last];
    chars[last] = chars[i];
    chars[i] = temp;
    i++;
    last--;
   }
   field.set(str, chars);
  } catch(NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
   e.printStackTrace();
  }
 }
}

// Question 3: Create the custom BlockingQueue to write the Producer/Consumer scenario.
// It took me 90 mins to write the below code about the idea I had.
// One can change the producer/consumer threads wait as per the requirements.
// It is just to represent the idea & not tested properly.
class BlockingQueue1<T> implements Queue<T> {
 private int defaultSize = 10;
 private int size = defaultSize;
 private T[] queue;
 private int push = 0;
 private int pull = 0;
 private int prodWait = 500;
 private int consWait = 500;
 @SuppressWarnings("unchecked")
 public BlockingQueue1() {
  queue = (T[])new Object[size];
 }
 @SuppressWarnings("unchecked")
 public BlockingQueue1(int size) {
  queue = (T[])new Object[size];
  this.size = size;
 }
 @SuppressWarnings("unchecked")
 public BlockingQueue1(int size, int prodWait, int consWait) {
  queue = (T[])new Object[size];
  this.prodWait = prodWait;
  this.consWait = consWait;
  this.size = size;
 }
 @Override
 public int size() {
  return Math.abs(push-pull);
 }
 @Override
 public boolean isEmpty() {
  return push == pull;
 }
 @Override
 public boolean contains(Object o) {
  return false;
 }
 @Override
 public Iterator<T> iterator() {
  return null;
 }
 @Override
 public T[] toArray() {
  return queue;
 }
 @Override
 public Object[] toArray(Object[] a) {
  return null;
 }
 @Override
 public boolean remove(Object o) {
  return false;
 }
 @Override
 public boolean containsAll(Collection c) {
  return false;
 }
 @Override
 public boolean addAll(Collection c) {
  return false;
 }
 @Override
 public boolean removeAll(Collection c) {
  return false;
 }
 @Override
 public boolean retainAll(Collection c) {
  return false;
 }
 @Override
 public void clear() {
 }
 @Override
 public boolean add(T e) {
  //  System.out.println("Got : " + e);
  synchronized(this) {
   while(queue[push] != null) {
    try {
     System.out.println("Waiting for Queue to have space....");
     wait(consWait);
    } catch (InterruptedException e1) {
     e1.printStackTrace();
    }
   }
   System.out.println("Got : " + e);
   queue[push] = e;
   push++;
   if(push == size) {
    push = 0;
   }
   notifyAll();
  }
  return true;
 }
 @Override
 public boolean offer(T e) {
  return add(e);
 }
 @Override
 public T remove() {
  T ans = null;
  synchronized(this) {
   while(queue[pull] == null)
    try {
     System.out.println("Waiting for Queue to have data....");
     wait(prodWait);
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   ans = queue[pull];
   queue[pull] = null;
   System.out.println("Removed : " + ans);
   pull++;
   if(pull == size) {
    pull = 0;
   }
   notifyAll();
  }
  return ans;
 }
 @Override
 public T poll() {
  return remove();
 }
 @Override
 public T element() {
  return null;
 }
 @Override
 public T peek() {
  return queue[pull];
 }
}
class Producer implements Runnable {
 private BlockingQueue1<String> bq;
 private int range;
 public Producer(BlockingQueue1<String> bq, int range) {
  this.bq = bq;
  this.range = range;
 }
 public void run() {
  System.out.println("Producer started.....");
  while(range > 0) {
   bq.offer(""+range);
   range--;
   try {
    Thread.sleep(100);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
  bq.offer("End");
 }
}
class Consumer implements Runnable {
 private BlockingQueue1<String> bq;
 public Consumer(BlockingQueue1<String> bq) {
  this.bq = bq;
 }
 public void run() {
  System.out.println("Consumer started.....");
  while(true) {
   String ans = bq.poll();
   if(ans != null && ans.equals("End"))
    System.exit(0);
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
 }
}