I can't hear you clearly.

我听不太清楚。

I can hardly hear you.

我听不太见。/我几乎听不见。

Say that again,please?

请再说一次。

Could you speak up a little,please?

可以请你说大声一点吗？

Does Mark Wyatt work there?

Mark Wyatt 在这里上班吗？

This line is terrible.

电话线路很差。

The line is dead.

电话线路坏了。

Could you spell that for me please?

可以请你拼给我听吗？

Have I got it right?

我说的对吗？

Got it?

好了吗？(听懂了吗？)