''는 것이었다'' is past tense of "는 것이다" 거 is a spoken form of 것 which means means "thing,fact" "는 거였어요" is past tense of "는 거예요" verb stem+는 거 is present tense of modifier verb stem+(으)ㄴ 거 is past tense of modifier 이 사과는 그가 먹는 거예요. This apple is that he eats. 이 사과는 그가 먹는 거였어요. This apple was that he used to eat. 이 사과는 그가 먹은 거예요. This apple is that he ate. 이 사과는 그가 먹은 거였어요. This apple was that he have ate. 그 전화 그한테 온 거예요. The fact is that the phone call was for him. 그 전화 그한테 온 거였어요. The fact was that the phone call was for him. 이 전화 그한테 오는 거예요. It is certain that this phone call is for him. All sentences of yours are correct.