Count number of instances

I have on a sheet a list of phone calls made from various extentions. Example below

Calling Party Dialed Number
2255 6700
2255 6700
2007 6700
2255 3456
2007 6321

I want to count the number of times each extention dials 6700. So the results from the above data would look something like this:

2255 = 2
2007 = 1

I've been playing around with COUNTIF but not getting anywhere. Can anyone suggest a function that may do the trick?

Excellent thank you that worked a treat!