How to fix "concurrent FAX session limit exceeded" error?
Asterisk cannot process additional fax calls until its fax licences are released from other (fax) calls, Usually you will see the following error in Asterisk CLI or log when this problem occurs.
[May 16 03:55:00] WARNING[C-00000017]: res_fax_digium.c:1604 dgm_fax_reserve: Cannot reserve FAX session - session limit exceeded (max: 1).
[May 16 03:55:00] ERROR[C-00000017]: res_fax.c:1887 receivefax_exec: Unable to reserve FAX session.
In order to fix this problem, you will need to purchase additional licences.
For more information of how to purchase fax licences, please visit Digium's Fax for Asterisk page.