Dahdi-linux echo/ directory has moved
The drivers/dahdi/Kbuild looks for the echo/ directory in drivers/staging/ but in recent releases that's been moved to drivers/misc/ instead in upstream.
The change to drivers/misc happened quite some time ago: https://github.com/torvalds/linux/commit/6e2055a9e56e292715f935a85f381e54c1f54269
What happened recently is that it was completely removed from the kernel: https://github.com/torvalds/linux/commit/7f4de1867ef9a787c618e6eb1540b34fef2643ae
So I think the right change at this point is to ignore the kernel directories altogether and include the removed drivers directly in DAHDI Linux.
Edit: Actually, I don't know if that can be done for licensing reasons, since this repository also has a dual license.