Thomson & Cisco modems - session timer

Hello,

I am looking for some help or advice. :)

My problem occurs when using Cisco EPC2203 & Thomson THG541 cable modems.

If these modems dial out and the connection establish, we send an INVITE after 900 s (this is our softswitch's session timer). The modem answers an OK, and for that it receives an ACK. After this the modem sends 4 or 5 five OK in a row, and for last a BYE.

Can You please tell us how is it possible? How can we fix this, so the call can continue?

Thanks in advance for every reply.