Despite having worked on telephone trunking in the 1990s and at the end of my career in Android system integration I really don’t know much about the cellular side of things work other than what I’ve read in the popular press.
MMS wasn’t working
This was brought home recently when I rediscovered that I was unable to send or receive multimedia text messages (MMS). I had not paid attention as I don’t normally use texting. But there was a situation that came up recently where everyone one else was communicating that way and I needed to be in the loop. I found that regular texts (SMS or short message service) worked fine. But even without actual media, group texts seem to be sent as MMS and I was not able to read them.
I got the notification, but my phone would return a message indicating a failure when trying to download the contents.
Since I am running a custom ROM there are a myriad of things that could have been causing this. Eventually I found that three things were needed for me to get them to work.
Weirdly, on my phone custom Marshmallow 6.0.1 there is a separate APN setting for MMS while on my spouse’s factory stock Lollipop 5.1.1 phone there is no separate setting. I assume hers uses the GPRS setting for SMS.
What about email?
For a long while I had noticed that receiving email while on mobile data was problematic. My best bet to get it to work was to enable my VPN. Fresh from my victory over MMS, I decided to take another look at this.
Watching the server email log file while trying things indicated that when on mobile data the phone wasn’t reaching the server at all. Nothing on the web indicates that T-Mobile US blocks email ports. But other web operations on the phone work fine. Maybe a routing issue?
Looking at the GPRS APN setting I noticed it was set for IPv6. In theory my mail server and DNS settings support IPv6 and while it has not been fully tested for POP or IMAP I have sent and received email via SMTP on IPv6. Tried changing the APN setting on the phone to force IPv4 and email magically began to work.
My VPN setup forces IPv4 so I think that is why email worked when it was enabled.
Obviously something is wrong with IPv6 in this setup. And it might be T-Mobile as after I got this far I did a search and found others had the same problem fixed in the same way.