MMS Messages Send from Stranger's Number

When I send an MMS (either a picture message or a group message), my recipients receive it from a phone number that is not mine. When they reply, their message goes to this other number, which belongs to someone who I do not know. If this happens in a group message, my own number is removed and replaced with the stranger’s number. The stranger is now included in my work group chat and my child’s softball parent chat.