Goodness knows what they've been up to—probably chewing through the phone cable.

天知道它们做了什么——很可能一直咬电话线。

It looks like a phone cable, like a regular old style phone cable that nobody uses anymore, okay?

它看起来像一根电话线， 就像一条不再有人使用的普通老式电话线，好吗？

Make sure means check - make sure that this is happening, in this case that it's hooked up to a phone line, meaning there is a phone cable connected to the machine.

Make sure 表示检查 - 确保这种情况正在发生，在这种情况下， 它已连接到电话线上， 这意味着有一条电话线连接到机器。