The text messages that I am trying to send using lumia 520 are converting to MMS. How do I stop this?
This could be happening for one of two reasons:
The more likely reason: you have
Group Text turned on.
Group text (in Messaging Settings) helps keep replies in one thread while you're texting with a group of people. When it's turned on, replies are sent as MMS (Multimedia Messaging Service) messages instead of SMS (Short Message Service) text messages. To find this setting, in Messaging, tap More > Settings. This setting can vary by mobile operator and might not appear. Your mobile operator might charge extra for MMS messaging, and this service must be available with your current phone plan.
The other possibility as Neil mentions in his comment to your question is that you are attaching some sort of media to the message that causes it to be sent as an MMS.
I don't believe that a really long text message would get sent as an MMS. I tried this out for upto 608 characters in a text and it would break it into 4 text messages (as opposed to one MMS).
Some Lumia phones are set to convert to mms after so many characters and some are not. It depends on what the service provider asks for when they buy in bulk. My wife and I have the same phone and hers sends multiple sms texts, where as mine converts to mms after 160 characters. We are both on different networks. Apparently on Lumia phones this cannot be removed, but there are other types of phone that it can be removed from. Also if you add characters like smiley faces and pictures etc, this will convert it to mms. This subject doesn't sit well with me, as the service providers don't tell you this when they tie you into a contract and start adding charges on for sending picture messages.