Q: When my friend sends group texts, she is identified on my iPhone 6 as an "unknown sender," even though she's in my contacts list. This doesn't seem to happen when she sends an individual text to me. What's wrong?
Gini Dodds, St. Paul
A: The problem may be caused by different kinds of texts.
If the sender and all the recipients of a group text are using Apple devices, the text will be sent as a group iMessage. That travels through Apple's servers rather than a cellular service's. Apple says its system is faster and uses encryption to ensure privacy.
If the sender or anyone in the group is using a non-Apple device, the text is sent via conventional text messaging, which uses either the SMS (Short Message Service) or MMS (multimedia messaging service) formats that are used by nearly all cellphones. Both are transmitted through the servers of cellular providers and are not encrypted.
Your iPhone should be able to correctly receive texts in any of those formats, provided those features are turned on. To make sure, go to Settings, click Messages, then look at the "on-off switches" behind iMessage, SMS and MMS — all should display the color green.
So, why is your friend being identified as an "unknown sender"? Here are some possible reasons:
• It's a message format issue. Your friend is correctly identified on one-to-one text messages (which I suspect are sent via iMessage) but not on group text messages (which I think are sent via SMS or MMS because at least one recipient doesn't have an iPhone.)