int i = 1;
            foreach (DataRow dr in dt.Rows)
            {
                 gkxx = new Proj_cfxfgkxx();
                 gkxx.lxdh = dr["联系电话"].ToString();
            }

List<string> temp_list = dt.AsEnumerable().Select(a => a.Field<string>("lxdh")).ToList();

------解决方案--------------------
gkxx.lxdh 是个List<string>吧？
C# code

  foreach (DataRow dr in dt.Rows)
            {
                 gkxx = new Proj_cfxfgkxx();
//填加到List里
                 gkxx.lxdh.Add(dr["联系电话"].ToString());
            }
//转成string加"," 
 string s = string.Join("','",gkxx.lxdh.ToArray());

//写你的SQL
select * from ---- where lxdh in ('" + s + "')"
判断就行了。