A few Datacentres I know have POTS/ISDN capability as they host a fair view VOIP companies. I assume at some point that VoIP call needs to get into the existing telephone network somehow. Also, companies would locate their telephony stack in a DC and route over ethernet just so the kit was more secure and in disaster scenarios they pick up anywhere.

Yup. Lotta places will now give you 1Mbps 95% on FastE inclusive with a service contract, if you ask nicely, and are taking a cage with 20+ racks. Datacenters which ask you to pay it's usually <$50 MRC.

Would guess the transition is slow because if it ain't broke why fix it, and its often a lot more than just provisioning a new circuit, you have to update process documentation, etc.

Lots of companies still have inbound dialin access for EDI stuff still running on POS systems from the 1990s. It's scary really.

It's also used for OOB access in high security environments in case you are getting strategically DDOSed.

Makes me wonder what you'd find wardialing in 2016.

I've used dial up via a cheap modem to a Cisco 2500 as recently as a few years ago for OOB console access. It's not gold plated but still works fine

More recently though it's all been on ethernet handoff either via the DC or a local provider