Spare iPhone WiFi Only iMessage Activation

Of course it cannot send SMS or MMS. Those are cellular telephone services and require an active account with a cellular service provider in a plan with texting included.

Your iPad Air is paired to your iPhone 6 via iOS continuity (Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support) and is using its cellular service to send and receive the SMS texts.

However, iOS continuity will not allow you to similarly connect an iPhone to another iPhone. That simply is not supported. So to use SMS on the 6+ you would have to activate cellular service on it. Maybe an inexpensive prepaid plan could work for your needs?

BTW, you also cannot initiate iMessage on an iPhone without SMS enabled first. When you signed out of iMessage on the 6+ you removed that device from iMessage and your AppleID. To put it back, you would need to have SMS texting enabled first so the iMessage activation service (which is different for iphones versus data only devices like an iPad or Mac) can complete the device registration with your AppleID (and your cellular telephone number).

Why you cannot connect an iPhone to an iPhone via continuity is not something anyone here can answer, other than to say because Apple has not built in that option. You cannot do so, that‘s all I can say on that matter.

When you restored the iCloud backup, was that backup made when the device had active iMessage service associated with it?

Now, to associate it anew with iMessage, it needs to be able to send the authentication SMS to Apple. That requirement is specific to iPhones and iMessage. Again, why is simply because that’s the way Apple has designed it.

But there is no way to set up a new iMessage account on an iPhone without that iPhone having cellular service and SMS texting enabled (If you get an error when trying to activate iMessage or FaceTime - Apple Support).

I doubt it. They would simply be able to activate it the same way you would. Apple store employees do not have admin or root access to Apple’s secure data systems so cannot step in and over-ride system wide processes like account activation and such.

I don’t know if the GSM side of Sprint iPhone 6/6+ are unlocked or not, or whether they will unlock it, but you could ask them about that. That phone will never work with any other CDMA based carrier, but if the GSM side is carrier unlocked than AT&T or T-Mobile or any of the many MVNO’s that use those twos networks would work.

There are also MVNOs like Boost Mobile that use Sprint’s network so maybe you can take your own Sprint device to them and get service, some sort of minimum cost prepaid plan with texting?

You had had said you wanted SMS and iMessage so the only way to get both on an iPhone is to set up cellular service on it. So I was suggesting something like an AT&T prepaid plan, or an inexpensive MVNO plan. Then you could have both SMS text and iMessage capability at the lowest service rate possible.