To keep everyone updated, CUBE and SIP SRST are now supported co-resident on the same ISR 4000 series router beginning with IOS XE 16.7.1. The key feature that allowed this to happen is SIP multi-tenanancy, allowing the line- and trunk-side SIP feature sets to coexist without stepping on one another. Additional detail is provided in the CUBE Configuration Guide Supported Platforms chapter and SRST System Administrator Guide Configuring SIP Trunking on Unified SRST chapter.
In my opinion, we're unlikely to see SCCP co-residency added in the future. My impression is that Cisco has consolidated all future development effort on SIP, likely as a pragmatic realization that they can't keep up by keeping all of the "legacy" protocols afloat. Again, that's only my opinion.
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