GTP echo response

Hi everyone, We're using kernel GTP developed by Osmocom on openair-cn. Turns out that we are not able to connect an UE for longer than 20 seconds using an ZTE eNB apparently because the GTP-U is not answering to the echo requests. As far I know this response is supposed to be implemented but it is not answering. Do you guys have any tips on this issue? Thanks in advance, Alandre