You have to do this by index rather than value:
>>> phone = {"first":100,"second":200,"third":[10,12,5,38],"fourth":400}
>>> del(phone["third"][1:3])
>>> phone
{'second': 200, 'fourth': 400, 'third': [10, 38], 'first': 100}
This deletes elements in position 1 and 2 in the list.