[Hooston]

If the weather is calm, it is late war and the escort has high proficiency then it can be very very difficult to shake it off. This is realistic. The conventional uboats became suddenly obsolete in mid 1943 and largely withdrew from the North Atlantic convoy routes. Things get really bad from 1944 onwards; attacking convoys becomes too dangerous because of the threat of multiple escorts equipped with hedgehog. The SH3 homing torpedoes help as they are somewhat OP, but it is time for your Kaleun to transfer to a training flotilla or head for the South Atlantic and only attack lone merchants.
Tonci87 runs through the usual evasion tactics in his latest "U-tube" video:

With a single escort Tonci's tactics should work eventually. The escorts only hang around for about 20-40 minutes after they have lost contact so it is important to stay silent as soon as you think they have lost you, even if they are still running in to attack the last known position. The same principle applies to multiple escorts, they each seem to have their own independent "lost contact" timer and leave one by one.

An escort will run out of charges eventually, giving you an opportunity to get up to periscope depth and sink it. Not easy with manual targeting. If the weather is nice you can also fight it out with the deck gun. Good luck with that.