This is not a random hacking issue. My boyfriend recently transferred his phone number from his family's Sprint account to my Verizon one. It's been about a month and his new phone works with his number like usual. Yesterday, his mother sent a group message to the family and it showed as coming from my boyfriend's number, even on his phone which did not send the text. Can this possibly be a fluke where the number is active on both Sprint and Verizon, or does it have to be some kind of spoofing situation?
What you described does sound unusual. Once everything is switched, it would only be active with Verizon. Are you receiving any bills from Sprint? If not, then we know it's not active with them. Tell me, is your boyfriend's mom also a Verizon customer? Have you been able to recreate this, and have his mom send a group message, and reply within the message once it's sent to the group?
Without knowing some additional details, we can only speculate. Please share some additional details for us, thanks.