Cancellation apps?

Hi, just wondering if anyone can recommend test cancellation apps that can reserve tests for you for a short while? My instructor wants me to check with him before I book a test to ensure he's free, which makes sense, but that means that every time Testi notifies me of a cancellation it's gone before he texts me back, even if it only takes him 5 minutes. Wondering if there are any that would reserve the slot for 10-20 mins just so I have time to check with him?