Always unreachable when dialling to a certain phone from my phone, but I can dial to other phones.

Reason 1: If you are dialing a regular phone (PSTN, cell phone, etc.), the problem may be with the service carrier. Contact your service carrier, ISP or agent.

Reason 2: If you are dialing an IP phone, the receiver may not be online or cannot receive the call. (Refer to Question 4 for details)