trigger ContactPhone on Account (After update) 
{
	List<Contact> lstConForPhoneUpdate = new List<Contact>();
	Map<Id, String> mapAccIdToPhone = new Map<Id, String>();
	
	for (Account objAcc : Trigger.New)
	{
		// if phone is changed
		if(objAcc.Phone != Trigger.oldMap.get(objAcc.Id).Phone)
			mapAccIdToPhone.put(objAcc.Id, objAcc.Phone);
	}
	
	// iterate over contacts of the accounts whose phone is updated
	for(Contact objContact : [	SELECT Phone, AccountId 
								FROM Contact   
								WHERE AccountId IN: mapAccIdToPhone.keySet()
								])
	{
		// assign phone
		objContact.Phone = mapAccIdToPhone.get(objContact.AccountId);
		lstConForPhoneUpdate.add(objContact);
	}
	
	if(!lstConForPhoneUpdate.isEmpty())
		update lstConForPhoneUpdate;
}

@isTest
public class ContactPhoneTest
{
	static testMethod void testForContactPhoneUpdate() 
	{
		List<Contact> lstContact = new List<Contact>();
		
// Don't forget to include required field value pairs in Account and Contact which I am inserting here.
		Account objAccount = new Account(Name = 'Test Acc', Phone = '12345678');
		insert objAccount;
		
		for(Integer i=0; i< 10; i++)
		{
			lstContact.add(new Contact(LastName = 'Test Con'+i, AccountId = objAccount.Id));
		}
		insert lstContact;
		
		// update account's phone 
		objAccount.Phone = '12345679';
		update objAccount;
		
		for(Contact objCon : [SELECT Phone FROM Contact WHERE AccountId =: objAccount.Id])
		{
			system.assertEquals(objAccount.Phone, objCon.Phone);
		}
	}
}