Dailing by extension is a behavior that our customers and staff are very accustomed to. I expect most customers who are coming from other phone systems, ie. Nortel, RingCentral, and others are used to providing their extension to outside parties to call in to them rather than a bunch of random DID numbers. The July 2018 roadmap indicates that extension calling is available now (https://skypeandteams.blob.core.windows.net/artefacts/Skype%20for%20Business%20to%20Teams%20Capabilities%20Roadmap.pdf) but this is not the case. At least not for external callers calling into an auto attendant.
Martin Gonzalez commented
Are there any updates?
Can Microsoft confirm this? Our project is in crossroad due to this functional deficiency.
While voice recognition dialing is fantastic, however, it may not be able to recognize non-native English speakers