overall, I wonder how compliance to this SSO would look like—is there room to comply with SMS and payments, but not with Voice and USSD?
you make a valid point. it may be hard to define core by channel, given there are plenty of SMS-only services (your standard bulk messaging campaigns) out there, and other voice-only services (a medical hotline) etc. Perhaps the SSO is about compliance to certain minimal capability within a channel (e.g. in SMS, the ability to do delivery reporting, among other things). In short, I would not see a problem with having a SMS-only compliant aggregator and that in some cases you would prefer to work with a specialist player that focuses on one channel (and perhaps has direct MNO connections) rather than a generalist aggregator that can deliver multiple channels but who perhaps executes through another 3rd party aggregator.
However, apart from channels, perhaps there are other services which are truly common across all M4D services (e.g. short / long code provisioning, reverse billing).