SUM function for time issue

Dear users,

I have encountered an issue and I can't seem to get a fix for it. English isn't my native tongue and maybe I can't seem to find the correct keywords to find a solution, however here it goes.
Basically I've made a Call Centre statistics excel where we store infomation about answered calls/missed, time when logged on, total time spent talking and so on. I have an application where I export the data I need (in to seperate excel file) and I just copy/paste (as values) that information in to my excel sheet and nearly everything works except one thing.

So the issue is a simple SUM function for time.

For example in the month November, I have pasted the total time "idle, working time, work status, post processing" from the other excel which was exported . However the SUM function on the G34,H34,I34,J34,K34 does not work. The time when pasted will be shown as "04:03:50", however if I remove the 0 in front and make it "4:03:50" the sum function will work. Is there a way to force excel to sum the time while value of the cell stays "04:03:50" or make a quick workaround to remove the 0's.

I have attached the excel file in question.
Please, tell me if I have to clarify, because it might not be easy to understand what I'm trying to say.