Custom PAM module for telnet

[stefano_20]

I have made a custom PAM module to manage "ad hoc" the telnet service. I observe that, when a client open a telnet session on my workstation, inside my custom module:
1) pam_sm_authenticate is called
2) pam_sm_acct_mgmt is called
3) pam_sm_open_session is called

When the client close telnet session by the exit command I observe that pam_sm_close_session is not called!!!
Can anyone help me?
In attachment there is the pam.conf file that I use.

Thanks in advance.