Continuos line tone!

Hi everybody !!

I already post this but in the wrong topic, my problem is this:

First, i have a bpx installed with elastix (july) distro, two Rhino R24FXX-EC Modular PCI Plug-In Cards (16 FXO and 32 FXS), 7 grandstream ip phones, and plenty panasonic analog phones (don´t remember the model but the simply ones with the 13 memory feature) plus HP040 faxes.
Everything was fine during the installation and basic configuration, but after that (this only happens in the analog phones) from time to time when you try to make a call either to an extension or external number you only hear the continuous line tone from the pbx when pressing the buttons, I discovered that when I call back to that extension (the one with the problem) you can make calls again also when I restart the PBX, this only happens with few extensions and includes 3 faxes HP1040, panasonic analog phones in total 8 extensions in diferent places of the building but always the same extensions.

Any ideas
Thanks for your time I would really apreciate your help.