What is "@" "at" called when giving an email over the phone? Is it different in different regions? I have heard it as "XiaoLong ..." or something similar to that in Taiwan, but not sure what exactly they are saying.
As pointed out by @Daniel Yeung above, they call it 小老鼠 (Xiao3 Lao3 Shu3）in Taiwan, which means 'little mouse'. Takes a little imagination to see the resemblance. I think that is precisely what you heard over the phone.
I've heard at least two versions personally.
One is to use the English reading of "at".
The other is "圈(儿)a". Meaning "a" with a circle around it.
It might be different in different regions.
我觉得没有什么特别的叫法，只是音译（根据发音翻译）一般叫“艾特” means @