Manually assign a PCI card to an interrupt

I've suffered these same issues for years now. As noted by Craig, name-brand motherboards tend to allow you to do less than some consumer motherboards regarding these controls. I've built all my systems using IBM xSeries, and it's been very hard to isolate the card on an exclusive IRQ. Latest firmware of some cards is even incompatible with some x3200 servers (looses lots of IRQ)

Try disabling all non-used devices (USB ports, parallel, serial, IDE if disks are SAS, planar RAID if not used, unused ethernet ports, etc.), this will free up some IRQ lines and provoke a recalculation. Otherwise, remove all cards and start from scratch, adding one card at a time. Sometimes you manage to find one, sometimes not.

Not that this will answer your question, but you could also replace one of the cards with some external device or other brand. Nowadays I buy Sangoma with much better results, I got tired of Digium and their problems with IRQs. Other options are external devices like Redfone (conntects over Ethernet) or Xorcom (connects over USB). These have E1/T1 interfaces. If looking for FXO/FXS ports, there are plenty of ATAs to choose from (try Audiocodes)

it depends on your motherboard BIOS.

some BIOSes allow you to assign a particular IRQ to a specific PCI slot. Some allow you to reset the PCI IRQ assignments (i.e. clear and re-assign all PCI IRQs) - this is often all that you need to do. some allow both. and some newer BIOSes don't seem to have any manual control over IRQs. NOTE: name-brand (Dell, HP, etc) motherboards are less likely than consumer/hobbyist (e.g. Gigabyte, Asus, etc) motherboards to allow manual control.

BTW, digium cards are pretty awful for interrupts anyway. 1000 interrupts per second even when it's doing nothing. if i were you, i'd start looking for alternatives (e.g. i forget the brand, but there are USB devices with multiple FXO/FXS ports). try searching [http://voip-info.org/]