SMS Sent Yet Never Received By Recipients

Since about a month ago, all text messages (SMS) sent from my number could no longer be received by any other number, including US and non-US numbers, despite the fact that all messages display as successfully sent on my end as usual. I did not realise the problem since nothing reflects oddly on my end and it was not discovered until I mis-scheduled an important personal event about a month ago due to no one having received my messages. Having contacted AT&T customer services and tested around by myself, these following facts deserve kind and thorough consideration before any blunt conclusions and/or helpless suggestions shall be drawn. 

1. Current plan & service: active service with AT&T Prepaid (since 2012) on plan - 45 USD, unlimited nationwide calls, unlimited nationwide messaging, unlimited international messaging, 6 GB data for smartphone; 

2. SMS text messages appeared as sent on my end, and then went missing in the cellular networks, resulting in being not received ever with no exceptions; 

3. MMS picture messages through mobile data can still be received successfully with no exceptions; 

4. Not attributed to phone hardware - tested on multiple mobile phone models, including but not limited to: Apple iPhone 4 (iOS 6.1.3 10B329), iPhone 5 (iOS 6.1.4 10B350), Apple iPhone 5 (iOS 8.4.1 12H321), Apple iPhone 7 (10.2.1 14D27), Blackberry Bold 9000 (Blackberry OS v4.6.0.304), Nokia N78 (Symbian S60V3 30.014.205.8) [N.B. all testing devices had been proved beforehand to be in proper working condition with other SIM cards; all tests performed with turning on and off the devices being tested]; 

5. Not attributed to phone software - tested on multiple mobile phone software platforms and versions (refer to above list of devices and software versions); 

6. Not attributed to SIM card - tested after switching to a new SIM card (new ICCID) from AT&T; 

7. Not related to data/Wi-Fi - tested under different data situations, including: mobile phone signal only, 4G, 4G LTE, Wi-Fi [N.B. all tests performed without internet messaging (when applicable (E.g. Apple iMessage))]; 

8. Not attributed to mobile signal strength - tested under different strength of signal (measured by bars shown on screen) (N.B. all testing signal strengths had been proved beforehand to be sufficient for other mobile phone users]; 

9. Not related to location - tested at different states within the US, including: GA, TN, NC, VA, WV, MD, PA, NJ, NY; 

10. Not related to recipients - tested by sending SMS text messages to multiple mobile phone numbers that are based either in the US or in other countries [N.B. phone and SIM cards of all recipients had been proved beforehand to be in proper working condition]; 

11. Not related to blockers by recipients - proved by a) calls between my mobile number and recipients (round way) work properly; b) all SMS messages can still be received; c) confirmed with carriers of mobile numbers of recipients; 

12. Not attributed to phone calls - all phone calls from and to my phone number are still in proper working condition, regardless of hardware models and software versions. 

And after multiple conversations over the phone with AT&T customer service, both the Prepaid department and the Wireless (post-paid) department clearly indicated that they do not see any problems with the service of my mobile number, and according to their response, they do not have any further means of testing nor solutions nor suggestions to offer (i.e. there is nothing they can do to fix the problem with my mobile number)! Nonetheless, it should be considered responsibility of AT&T to fix the problem, for SMS text messaging is a basic and fundamental paid service included in the service the customer of the mobile number service purchased from AT&T. In other words, the transaction is not completely fulfilled if the service cannot be successfully delivered. Therefore immediate help and support from AT&T is demanded and the states problem shall be attended to carefully. However again, please do not suggest or conclude before having carefully reviewed aforementioned facts from tests.

Thank you very much. 

Remarks: 

1. Deserting the current mobile number is not a solution; 

2. Strangely the SMS messaging worked for once, which was the very first SMS text message sent out after switching the SIM card (sent to the customer service representative at the store who helped with the switching of the SIM card, yet then never worked again); 

3. It appears that all other services for my mobile number is in working condition (E.g. data, hot-spot tethering, calls); 

4. Suspicion: is it possible that the "Unlimited" messaging is just a so-called term and set as a large number in the AT&T backend system (E.g. 1000000, 999999, 2^N)? I do send a very large number of text messages, especially in the past few months.