I have 8 frame-relay circuits 128/64 cir and some of them are using ISDN as a backup solution.
Sometimes all the frame-relay links get down and the backup doesn´t work because the serial interface keeps up and down all the time.
When it happens the service provider technical support usually asks me to press the RDL button in the modem and the frame-relay links works again.
They have already changed the modem.
May the problem be with the router hardware? If I issue a shutdown in the serial interface the backup works fine.