不知道你请的人的身份，也不知道是什么活动。
但以下这几句肯定是用得到
1.问候
Good Morning/afternoon, Mr./ Ms. XXX (May I speak to Mr./Ms. XXX (if the call needs to be transfered by an operation desk))
2.自我介绍
This is XXX calling from XX Investment Bureau.
3.说明致电目的
We would like to invite u to the forum/conference/ceremony on XXX(领域)
4.简要介绍活动
This event is organized by XXXX （谁主办）for XXX（什么目的）
It's a half/full/2-day event on Month/Date（时间）in Place(地点）
5.请人干嘛
We hope you can XXX(give an openning speech/a key-note presentation)
6.会议费用
Thie meeting/forum is free for participation.
7.结语
Thank you very much for your time. I will fax/copy you the meeting agenda shortly. If you have any question, please call me or email me.
Have a nice day.