iptel/voxalot/sip2sip

http://www.osslab.org.tw/VoIP/IP_PBX/%E8%BB%9F%E9%AB%94%E5%BC%8F_IP_PBX/Asterisk/Tips/%E5%90%84%E5%AE%B6_Provider_%E7%9A%84_SIP_Trunk_%E8%A8%AD%E5%AE%9A