在SIP客户端和模拟/移动电话之间构建VOIP电话

I've Built a VOIP Network for my House using Asterisk as server and SIP softphone as client. Everthink is going good and i can call all SIP client of my VOIP Network. Now I have no idea how to call an extern mobile phone or analog phone . I've heard something about Gateway to access to another network.I've Built a VOIP Network for my House using As