Bad incoming text numbers (Can't reply)

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This issue has only been happening recently, but I don't know what to make of it. All of my contacts are stored as 10-digit phone numbers on my phone (area code+7-digit phone number). I don't have any SIM contacts or Google contacts. For some reason, while I can text everyone just fine using the stored numbers, *sometimes* replies come from completely malformed numbers. For example, if I send a text to 123-123-1234, I'll get a reply from 123-312-31234. All my replies will come from the number with the random extra digit mixed in, but I can only send messages to the original 10-digit number for them to actually be sent. This has never happened to me with calls so far, just texts. What could be causing this?

*bump*tenchars