Ugh. No fun there. I still make a ton of phone calls, and I had a screen problem that made it go blank as soon as the call connected on my L900. So, I couldn't end calls or do select numbers to answer automated operators or dial extensions. Just a pain.
Is it just after the update to WP 8.1? This is a new problem if so. If not, you may need to give Nokia customer support a ring on this one, instead of kicking around forums. They are excellent at providing help, our forum just glows with enthusiasm after dealing with them. If it was fine before the update, maybe this is a side effect of a low RAM device? I am just guessing here, but it sounds like it is feezing at a random point. May be just a glitch from the update on your L525. Can you go back to WP 8, and try the update again? A pain, I know, but may be the easiest way around this.