Why do other people on my plan receive texts that I send to certain people?
Are they part of a previous "group" message you sent? You may be inadvertently sending to a group rather than the individual.
You posted this in the Windows phone space - but this has come up with iPhones if two or more people share the same Apple ID. Perhaps you are sharing the same Windows ID? What messaging app are you using?
The group message thing had me confused for a while...I finally figured out that if I sent a MMS message to four people, and three replied using the reply option in the actual message, everyone who got the first message gets all the rest...
