I've been sporadically missing text messages and think I've finally tracked down how to fix it. There are 2 main actions you need to take:
1. Configure your texting app to use MMS for "long" messages. I use Textra as my texting app, and highly recommend it. Its hard to narrow down the specific failure, but I believe when a text message is larger than 160 characters, it is broken up into multiple messages, and then needs to get reassembled on the receiver phone. There seems to be a failure somewhere along this process.
2. Configure all your APN protocols to IPv4 only. I've had problems with IPv6 and IPv4/IPv6 settings, especially connected over wifi to a router. I believe IP6 introduces issues depending on client/server implementations. On my Nexus 6, this is under Settings > More > Cellular networks > Access Point Names. Select each APN listed and set "APN protocol" and "APN roaming protocol" to IPv4 only.
Unfortunately, this requires changing configuration on both the sender and receiver of the text, which you may not have control over. But fixing one side is hopefully better than fixing neither...
These are the observations and notes I've noticed so far:
1. I only seem to be missing "long" messages, messages larger than 160 characters. These long messages need to get broken up and reassembled, and thats where a failure seems to occur.
2. I have seen this issue reported mainly with Android phones. I believe iPhone defaults to the settings above, but I don't have an iPhone to verify.
3. The APN protocol seems to be defaulted to IPv6 on many phones - including Nexus 6, Motorola X Pure, and Samsung Galaxy 6 I've seen so far.
4. I've seen a lot of threads where tech support says update your phone or update your SIM card. This does not make sense to me. This issue happens to many different phones, and ones that are relatively new.
5. I missed texts mostly from my wife, who is also on T-mobile. So being both on T-mobile doesn't seem to help.
6. Textra is a great texting app and can help with these missing text messages. I have mine configured to convert Long SMS to MMS when > 1 SMS (I assume thats the character length of 1 SMS, versus 2 or 3, etc). Also, you can configure it to show text status - Sent and Delivered. Although, when sending a MMS message (or a "long" SMS which is converted to MMS), I never seem to get a Delivered status; yet the message always gets delivered. FYI.
Please let us know if this works for you. If not, please let us know your:
* Did both the sender and receiver have the 2 settings above configured?
* Phone, OS and version, and Carrier for both the sender and the receiver. e.g. Nexus 6, Android 6.0.1, T-mobile
* At the time the text was sent, for each the sender and receiver, was the person connected to wifi? Did both the sender and receiver have a "good" cellular connection?
* Any other relevant information so we can narrow this down...