"Your SMS mailbox is full. New messages cannot be received until you delete some messages" - The unresolvable issue

This issue has been an absolute nightmare, spending countless hours online, on the phone and visiting locations with nobody being able to resolve this problem. I think i've spoken to 6 online chat agents, 2 phone agents and visited both my carrier and an apple location, within the past 3 days, with no luck.

I received this error message on Saturday and since then have been unable to receive any SMS messages. iMessage was working completely fine until one of the technicians got me to do something which caused iMessage to stop functioning, which now is stuck with the "Waiting for activation.." message, which is unable to activate due to the requirement for functioning SMS capabilities. Storage on my phone was also not an issue, I initially had 10.5GB available when the problem first started, and was able to clear up to almost 25GB of available space, which also did not help.

I've tried everything. All of the typical tricks for iMessage activation, I've deleted all of my text messages and threads, I've gotten a replacement phone, I've restored to factory settings and restored from computer backup, i've restored to factory settings and restored from iCloud backup, i've reset network settings, I've tried everything and followed every direction that all of the 10+ people i've dealt with have recommended, and nobody is able to correctly explain or troubleshoot the issue. To my understanding now, there simply must be a software issue within iPhones themselves, deeming this error message unresolvable from a consumer's perspective.

This issue needs to be seriously addressed to the programming team as now myself, plus many other stuck individuals I have read about online, have lost all of their content and messages stored on their device

Currently running iOS 9.2.1 on iPhone 6, 64GB