Call placed on hold "undesidered"

Hi all,

on a system S412 (Fw version 30.14.0.23) I have a strange behaviour, sometimes during the calls by the FXS extensions with the BRI trunks suddenly the external party is placed on-hold and the analog extension receive the dial tone that allow to digit a number of an extension to transfer the call.
Obviously this behaviour that I've just described would be normal if it is required by the customer, the problem is that the customer hasn't place on-hold the call...
Sometimes the call suddenly  come back on-line otherwise the customer have to mandatory manually retrive the call.

Did anyone has encountred this problem very-strange?

P.S.: What I've just described happen only on the FXS extensions.