Can't text a contact on my iPhone 4

I just found out that I can't send a text message to a friend. I had them as a contact, they are on Verizon, and I can receive texts from them, but they get nothing from me. I tried deleting the contact and just sending to the number directly, but to no avail. Has anyone else experienced this? I searched for this issue, but didn't find anything.

JamesMB said:

Are you sending a plain text (SMS) or MMS?
Are you able to send texts to other people when this happens?
Do you get the symbol that shows that the text failed, or does the text appear to send ok from your phone?
What happens when you place a voice call to the same number?

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They are plain SMSs, not MMS.
I am able to send texts to other people at all time.
I don't get any symbol to show that it hasn't sent, it looks just like a sent text message.
I've never tried calling it, I'll give it a shot and see what happens.