[Motorola Photon 4G] bitbang3r:
As others have mentioned, if you have VoIP service independently of Sprint, there's nothing to stop you from using it on your phone. You can do a complete end-run around Sprint and cobble your own adhoc SVDO-ish work-around today. Your battery life will totally suck, and you'll be paying more money for VoIP from some thirdparty provider, but you can do it. Just run the client, forward your calls to your VoIP number, and you're in business.

So, why can't SPRINT do it? Simple. The service we all call "Call Forwarding" is heinously inefficient. It was invented ~25 years ago, and only works for its intended purpose (adhoc forwarding of calls to arbitrary numbers) because there are just a few people using it at any given point in time. If EVERYBODY (or even a large plurality of Android owners) tried doing it at once, Sprint would hit a hard limit beyond which no more users serviced by a given switch could forward calls until somebody else discontinued forwarding to free up a circuit (remember, ESS voice calls are circuit-switched, regardless of any packet-switched magic that might occur at the backhaul and trunk level. You'd be amazed how much metaphorical WD-40 and duct tape still gets used behind the scenes at Sprint, Verizon, AT&T, and T-Mobile to glue 21st-century wireless and 19th-century wired phone circuits together & make wireless calls look like landline calls to the PSTN).

To implement large-scale SVDO that's cost-effective, scalable, and sustainable, Sprint has to make much bigger changes that involve their switches (enabling them to terminate calls directly to VoIP instead of relying on adhoc ESS-type call forwarding) and signaling (sending notifications via the same mechanism used now to notify the phone of incoming calls, deliver text messages, and carry Google & Apple's push notifications). That way, instead of forcing your phone to establish and maintain an EVDO or LTE connection 24/7 and use it to poll for incoming calls, it can still use the more power-efficient 1x method, and simply notify the onboard VoIP app that there's an incoming call when it happens.

At the end of the day, it's one of those cases where it's easy for nimble, tech-savvy, and highly-motivated individuals willing to spend a few days learning and experimenting to take matters into their own hands and hack something into working for themselves, but it's a much bigger job to make it viable for the unwashed masses who want it to "just work" transparently without disclaimers, gotchas, and devastated battery life. It's the same reason why iPhones don't have wimax on Sprint. Apple point blank refused to even consider implementing something where users have to toggle it manually and be aware of their connectivity options at all times to really make it work right. Think I'm kidding? Find a nontechnical Sprint customer with wimax-capable phone, and check out their 4G settings. 95 times out of 100, they'll have 4G disabled, even in cities where there's been 4G service for more than a year. They went to the Sprint store, complained about not being able to "surf the web" (WE know their phone was probably thrashing back and forth between 3G and 4G & kept spending 5-20 seconds at a time in "no network connection" limbo), the employee toggled 4G off, handed the phone back to them, and they never had any clue what the employee actually DID. They were just happy that their phone quit endlessly saying they had no internet connection. ;-)