I'm trying to format cells in Excel so that they display the total duration of phone calls as hh:mm:ss, but Excel is giving me errors.

Sometimes durations are only mm:ss (49:10), or even just ss (35), and I need them by default to change to 00:49:10 and 00:00:35 respectivly. However, when I select 'Custom' on the 'Number' tab when formatting the cells and enter either 00:00:00 or ##:##:##, Excel tells me -

Microsoft Office Excel cannot use the number format you typed.

Also, hh:mm:ss will not work for me, as I'm dealing in durations, not times. Is anyone able to tell me how do format this? Thanks.