Texting. Every time I use a zero it changes to just numbers!!

When I do a text that includes a zero it changes all the keys to just adding numbers only. I can get it to predictive text sometimes, but I don't want that, just the usual ABC option. This is really annoying. I do a lot of competitions using texts and every single time I use the number zero it changes in this way. Hoe do I stop this and how do I change to simple ABC? I don't want this to keep happening and the instruction manual doesn't mention it at all