Excel not processing times in a way that can be used to calculate totals

I've received data from a 3rd party regarding incoming calls to my organisation.

I've sorted the data and tried to use it in a pivot & Power BI but neither recognise the call duration time.

I've tried formatting the cells to a standard time as well as several custom ones but nothing helps.

The only way I've found to get it to recognise it is by retyping it, but i have several thousand entries and I'd, obviously, prefer not to rewrite them all by hand.

Has anyone else had, and hopefully, solved this issue?

Details:

The format is 00:00:00.

Custom formats tried: hh:mm:ss, mm:ss, [h]:mm:ss, [h]:mm