1. Scenario:

1. Explanation : 1.

   List<Account> accList = [SELECT Id, Name, Phone, (SELECT Id, Name, Phone FROM Contacts) FROM Account WHERE Phone != null AND CreatedDate = LAST_WEEK];
   
   		List<Contact> conList = new List<Contact>();
   
   		if (!accList.isEmpty()) {
   			for (Account acc : accList) {
   				if (!acc.Contacts.isEmpty()) {
   					for (Contact con : acc.Contacts) {
   						if (con.Phone == null) {
   							con.Phone = acc.Phone;
   							conList.add(con);
   						}
   					} 
   				}
   			}
   		}
   
   		if (!conList.isEmpty()) {
   			update conList;
   		}
   

   b.

   List<Account> accList = [SELECT Id, Name, Phone FROM Account WHERE CreatedDate = LAST_WEEK AND Phone != null];
   
   		Map<Id, Account> accIdToAccountMap = new Map<Id, Account> (accList);
   
   		/* Altearnate, you can use, 
   		Map<Id, Account> accIdToAccountMap = new Map<Id, Account> ();	
   		for ( Account acc : accList ) {
   			accIdToAccountMap.put(acc.Id, acc);
   		}
   		*/
   
   		List<Contact> conList = [SELECT Id, Name, Phone, AccountId FROM Contact WHERE AccountId IN:accIdToAccountMap.keySet()];
   
   		if (!conList.isEmpty()) {
   			List<Contact> conToBeUpdated = new List<Contact> ();
   			for (Contact con : conList) {
   				if (con.Phone == null) {
   					con.Phone = accIdToAccountMap.get(con.AccountId).Phone;
   					conToBeUpdated.add(con);
   				}				
   			}
   
   			update conToBeUpdated;
   		}