Hmm, what? I think e.g. Talk/Hangouts and Voice are radically different: one is about text messaging and video calls, another is about a connection to the "legacy" voice telephony network, with a dial-able number, etc. They started (long ago) form very different areas.
They could have a seamless bridge between them, though. They sort of tried: you can dial a phone from Hangouts, but not back, or you can receive your Voice text messages in the email, but not send a reply back (afaict).
Google is notoriously bad at cohesive product management. They have a number of bright ideas, but no visionary in the consumer product space, so apparently no coordination.