One thing that still confuses me about ISDN is that despite most of it being standardized in a series of ITU specifications, I see a lot of phones and other TE'

One thing that still confuses me about ISDN is that despite most of it being standardized in a series of ITU specifications, I see a lot of phones and other TE's advertise support for vendor specific protocols and other standards (National ISDN protocols, special 5ESS protocols and later things like DSS1) What I don't get is the signalling is all standardized (LAPD, X.25 or maybe Frame Relay at layer 2, and Q.931 at layer 3) and the actual data should just transparent to the protocol and only relevant to the devie function (I imagine a typical voice call would just alaw or ulaw data). So unless there's something going in the upper layers of an established call, why the need for various proprietary protocols that from the sounds of it are incompatible. There are devices and tools to simulate this proprietary protocols, so I suppose in theory I could reverse them, but they’re either rare to come by or very expensive for a random hobbyist such as myself.