IPhone 4S not receiving regular text messages?

I have an iPhone 4S and today, out of nowhere i just stopped getting texts. IMessage continued to work just fine, but regular texts were not being received. I used Facebook messenger the rest of the day in place of it, and people told me they had received all my texts, and replied, but i never got any replies. IIm still using my phone to make calls and it is working just fine. The only problem is receiving messages. I've already rebooted several times, reset network settings, deleting long conversations to free space, everything. I don't know what else to do.:/ I do have a software update available, but it has been available for quite so time and my phone has worked just fine without it. I haven't update because i don't have nearly enough room for that. I could delete every single picture i have and still not have room for it. At this point it isn't even an option.

Dunno

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