Owning a mobile phone definitely makes life more convenient,but limit your conversations to urgent ones and save the per

Owning a mobile phone definitely makes life more convenient,but limit your conversations to urgent ones and save the personal calls until you are at home.
这句话我怎么看不懂呢?帮帮分析下语法吧!
这句话我想到的翻译是：
拥有一部手机确实可以使日常生活更加的方便，但它仅限于紧急事件的通话以及你不在家保持与你的联系。
我是这样看这个句子成分的，不对的请指出来谢谢。
but limit your conversations to urgent ones and save the personal calls until you are at home.
中却主语，我把to urgent ones 以及until you are at home.做主语。这样就感觉好翻译多了，to urgent ones 以及until you are at home 能做主语？
to urgent ones ：是动不定做主语？可语法上说不过去啊,它不是动不定啊
until you are at home ：是主从句吗？那Until能引导一个句子成为名称从句？
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