Which meaning of 意见 in used in 他经常到客户不听取意见 and 如果有人愿意听取客户的意见?
Here it means suggestions.

From 新汉英大辞典:  

意见 [yì jian]
1.
（看法或想法） idea; view; opinion; suggestion:
a clash of opinions;
意见冲突
make suggestions for revision;
提出修改意见
2.
（反对或不满意） objection; differing opinion; complaint:
I have a lot of complaints about her.
我对她很有意见。
If there is no objection, the agenda is adopted.
如果没有意见， 议程就通过了。


BTW: 客户不 should be 客户部.