In China, the cellphone system is different from other countries.  There is no (or very limited) voicemail, and phones are paid for on a pay-as-you-go basis.  When you call somebody, you can pick up a lot of clues from the message/ring.

你好，你所拨打的用户正在通话中。  If this rings twice and then you get the message, then your call has been rejected.  If the message starts with that, then the person really is on the phone.

你好，你所拨打的号码已关机.  The person you called is either out of battery or has turned off his/her phone.

您所拨打的用户已停机。 The person is out of money on his/her phone.

您所拨打的用户无法接通.  The person you’re calling is out of service.  Maybe they’re in an area with no reception.

您拨打的号码无人接听.  Nobody has picked up, either he/she is not near the phone or is just ignoring the call.

Words:

拨打 bo1da3 – dial (of a phone)
用户 yong4hu4 – user
通话 tong1hua4 – to be engaged in talking on the phone
中zhong1 – when used as a suffix, it can mean that someone/something is in the process of completing the action
号码 hao4ma3 – number
已 yi3 – already (in the aforementioned cases, it refers to something that has happened before you called)
关机 guan1ji1 – turn off (the machine- cellphone here)
无法 wu2(3)fa3 – no way/unable
停机 ting2ji1 – stopped (the machine)
接通 jie1tong1 – to connect for talking