They are all right. Just one more thing, 给手机充钱 is slang. You can say it, but it is not very beautiful to write into an essay (unless you want to describe some common speech in every day) or something like that. 充值 is suitable for writing.

我给手机充值。 我往手机里冲钱。 我给手机充了100块。 给自己的手机冲钱/充值。 我帮我妈妈的手机充值/冲钱。 我到营业厅去给手机充值。 我买了一张手机充值卡。 我买了一张100元的手机充值卡。 我捡到一张面值500元的手机充值卡，哈哈哈哈......哎，是已经冲过的了！

Of course! You can also put the exact number of how much you are planning to spend behind ”充" for example: 给手机充100块钱。

可以說的。 另外說法“充值”

不过充话费比较直观呢