Duration Troubles

Hey,

I'm trying to calculate and filter duration colum,

My formula is: 

CALCULATE(COUNTA(CTI[end type]), FILTER(CTI,CTI[end type]="ABANDON"),FILTER(CTI,CTI[wait time]<"00:00:15"))

There is a problem with the last filter, I want to filter calls that their wait time duration is shorter than 15 sec, can you figure out what is the problem with the formula above?

Thanks