Google Voice let users make free calls to most places in the US and Canada, but as of June 1st, 2018, all outgoing calls, including international calls, will go through Sprint and be billed accordingly. Messages, calls and voicemails after that date will no longer be stored on Google Voice, and you won't be able to use Voice-enabled features like call forwarding, voicemail transcription and spam detection.
Google recommends disabling integration before the cutoff date of June 1st, and re-blocking any phone numbers using the carrier's MySprint application. You'll have to re-install Google Voice on your phone if you want to keep using it. (Check here for Google's complete article on the matter.)
Not all is lost. You can still use Voice by getting a dedicated number from Google, or even port your Sprint number to Google Voice, though you'd then have to get a new Sprint number. You'll never regain the handy centralization of all your telephony and messages across multiple devices, though. It also marks the end of unprecedented cooperation between a US carrier and tech company, entities that are normally oil and water.