Iphone app that will count up or down with voice
I found this app "Voice Over Timer" which looks to be configurable to do close to what you want. 

AppStore Link: 
https://appsto.re/us/TeCFM.i

edit upon further review, it looks like the countdown can only be announced for the last ten seconds. The AppStore page mentioned a customization but it is a customization from among pre-determined options.