SMS message may get converted to MMS message in the following situations:
1) When sending a long SMS message
In some devices the message type gets converted from SMS text message to MMS multimedia message if the message length exceeds the device-specific setting which is typically 5 - 10 SMS submessages (around 760-1520 characters). If the message is longer, it is converted to MMS to avoid potential issues in handling long text messages in the recipient device. The cost of a SMS message consisting of several concatenated submessages may also be higher than the cost of a single MMS message.
The conversion from SMS text message to MMS multimedia message occurs when the automatic message type is selected in the message options. This is the default mode where the device tries to automatically select the best mode for the message.
The conversion from SMS to MMS can be avoided by selecting Options > Sending options > Message type > Text when in the Text message window writing the message. The message is then always sent as SMS text message and no conversion to MMS occurs.
The maximum number of concatenated SMS submessages can be up to 10 (around 1520 characters). In multimedia mode the message length can be longer.
2) No phone number defined in user contact informationIf sending SMS to one or more recipients and one of the potential recipients has only email address and no phone number, the SMS will be converted to MMS. This is independent of the number of the potential recipients.In order to avoid the conversion to MMS, remove from the recipient list the contacts which don't have a phone number defined.