These may be coming from someone who got on the VZW website and used "Send a Text Message" to send these to you. I used to mess with people using this feature. Most carriers have this same option when sending from the web. However, it does not require you to enter in a return address or a name in the from field. Therefor, you are unable to respond.
I have received a couple text messages that have no reply number. How do I know they have received them?
I'm confused on your question, if you received a message without a reply number then how did you reply to them at all? can you explain this a little further?