Admittedly DISA is rare these days, but our primary need for it is to allow external access to internal (non-DID) numbers AKA tieline numbers served via our terrestrial microwave system.
And yes, there's an awful lot of grey hair around here, myself included. We also realize that Cisco is not going to succumb to our "special" needs. That indeed was one of the true beauties of the Mitel - they were (are) a very small company (comparatively speaking) and were willing to "cater to" some of the more reasonable whims of what to them represented a major account. In the late 90s ours was the largest single deployment of an all -48DC system in the US running expanded peripherals & flexible dimensions. At the time we were all DC powered we also (and still do) provide dual redundant -48 60-amp DC feeds to the LEC and IXC equipment bays (OC-48, OC-12 and OC-3) along with 5 shelves of DS-1, most of which gear stands idle today, as technologies -and requirements- change.
So far - knock on wood - (3 years into this) the Cisco CUCM (and UCXN, UCCX and CUPS)has proven itself far more reliable than we expected and early issues were determined as proving to our ntwk engrs that the network design wasn't really as robust as they all swore it was

- of course proving once again that data is far more tolerant of network SNAFUs than voice. In their defense Cisco offered to perform a full network analysis before the VOIP rollout, but their offer was perceived as an opportunity for them to try to up-sell us on a lot of gear we really didn't need then (and still don't). The only thing we did do was replace the two aging 6500 cores with a pair of 7K's & upgrade all the closet switches to 3560-G and beef-up the electrical service, replacing 20a circuits with 30a.
The culture shock was probably felt more by people like myself who have to administer the thing & having it dropped in our lap & then trying to learn to navigate a GUI programming interface after being accustomed to 25+ years using a CLI. Building the first few phones felt a lot like building sand castles using only tweezers and a magnifying glass. - i.e., so horribly granular. Of course all was LDAP-synched + requiring CSF softphones for each user in addition to their hard phones, enabling CUPS licenses, voice mail, etc.
We just last month finally got around to completing a forklift replacement of our E911 system, which up until then was still running on 1 remaining node of the Mitel. We had looked at CER with some functionality add-ons from a 3rd party custom configurator but decided that was too much of a cluster-xxxx and so went with a single-vendor solution (Enterprise Alert from AMCOM) which includes dynamic tracking of the moves/adds/changes down to the switchport level literally as they occur. (Several military installations also use Amcom) Our Cisco VAR came in and looked at it today & could not believe all that it does for 1/2 the cost of the solution they had proposed.
Original MUG/NAMU Charter Member