Using the app on your phone wont give you the info you require as you have no clue how your line is actually routing to the cab it's not uncommon for it to be longer than you think some even go in the opposite direction before looping around to cab it all depends what was around when the phone line was installed.
The only people who have the info to hand on how the line is actually routed are Openreach and therefore the best way to get what your actul distance is.
I was lucky when i had a fault a couple of years back the engineer showed me the route my line was taking and was able to give me the distance of 485m which was shorter than the distance measured by just walking to the cab which gave around 510m
