▲最佳答案应该是 by。对话的句意为：

——你最近跟安德鲁联络过吗？

——只是打过电话。

Only by telephone 是 We have only been in contact by telephone.

▲补充例句：

I shall contact you by telephone. 我将打电话与你联系。

You can order tickets by telephone. 你可以打电话订票。

Can I contact you by telephone? 我能用电话和你联系吗？

The interview was conducted by telephone. 采访是通过电话进行的。

You may place your order by telephone or on the Internet. 你可以电话订购，也可以网上订购。

Whatever city or town you may live in, you may reach us by telephone. 无论你住在什么城镇，都可以通过电话与我们联系。

It will save time if you inform him by telephone instead of (by) post. 假如你打电话而不是写信来通知他，你会节约更多时间。

▲注意：如果用介词 over，则通常有冠词。如：

I don’t want to say it over the telephone. 我不想在电话上谈这件事。

People are interviewed over the telephone. 人们通过电话接受了采访。