I never worry too much about the customer who chose a non-redundant system. They knew the potential perils going in and they freely chose the cheap option. In so choosing they essentially said, "I can do without my phones". Similarly, the "time and material" customer concerns me not. We'll get to them when we get to them.
On the other hand, Most Mitel gear is pretty darn reliable and the only moving parts are the disk drives. No matter where you or the customer is, FedEx can be there overnight and over a weekend you can always put something on a plane and have it met at the counter.
Going back 10 years I honestly cannot rember the last unplanned outage that wasn't either a hard drive crash or the result of lightning or "self-inflicted" by something the user did. The odds of losing a MCIII main controller or a peripheral switch controller or even a PIC card or a power supply are slim. Ground it, put it on clean power with a good UPS in a clean, temp-stable location and the dang thing will almost run forever.
On systems in hospitals or public safety (police, fire, ambulance, etc), they first need to be (must be) control-redundant on either DC power or generator-backed commercial UPS systems. On-site I would keep 1 spare hard drive, 1 spare DNIC card, 1 spare trunk card (or pre-programmed PRI card) unless my shop was in the same community or within 100 miles.
In your case, located on 1 coast and trying to support systems across the USA I'd probably hit the secondary market and buy an entire system (or two). A couple of complete main control cabinets and a couple of per cabinets, couple of DSU nodes, some phones, etc. That's by far the cheapest way to lay in a thorough supply of spares, altho the drives would be of questionable age. Still anything that'll spin up is better than something that won't. Just watch the secondary mkt stuff as some of it is older Mfg. Disc items.
