Service Cloud Voice is Salesforce's own product: calls, transcripts, and AI-generated summaries all live natively inside Salesforce records, built primarily around Amazon Connect as the underlying telephony infrastructure, with support expanding to other partners over time. It's the deeper option when Salesforce is meant to be the single source of truth for every interaction, voice included.
Open CTI is older and more general: it's a JavaScript API that any telephony vendor, Twilio, RingCentral, Five9, NICE, and others all have certified Open CTI implementations, can build against to embed a softphone inside the Service Cloud console, with click-to-dial, call controls, and screen pops. It doesn't lock you into one telephony provider the way Service Cloud Voice's Amazon Connect foundation does, but it also doesn't go as deep on native transcript and AI features.