How do you intercept instant messages on someones cell phone

Update: my ex was able to get into my cell phone messages from his cell phone and get a message that was imed to me how did he do that?
Update 2: yes we do share the same plan he has a Q9 smart phone and i have a lg both cinglular service and there is a second Q9 on the plan they both have the gps which i do not. we all have instant messaging
Update 3: he did not go through the phone company but he is a techy
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