My phone will not send SMS...only MMS.

Try this easy steps:
Restart your iPhone.
Check your network connection. To send a message as iMessage or MMS, you need a cellular data or Wi-Fi connection. To send an SMS message, you need to have a cellular network connection. If you turn on Wi-Fi Calling, you can send SMS messages over Wi-Fi.
Check with your carrier to see if the type of message you're trying to send, like MMS or SMS, is supported.
If you're trying to send group MMS messages on an iPhone, go to Settings > Messages and turn on MMS Messaging. If you don't see an option to turn on MMS Messaging or Group Messaging on your iPhone, then your carrier might not support this feature.
Make sure that your device has enough space to receive images and videos.
Make sure that you entered the correct phone number or email address for the contact.

Try this steps:

Restart your iPhone
Check your network connection. To send a message as iMessage or MMS, you need a cellular data or Wi-Fi connection. To send an SMS message, you need to have a cellular network connection. If you turn on Wi-Fi Calling, you can send SMS messages over Wi-Fi.
Check with your carrier to see if the type of message you're trying to send, like MMS or SMS, is supported.
If you're trying to send group MMS messages on an iPhone, go to Settings > Messages and turn on MMS Messaging. If you don't see an option to turn on MMS Messaging or Group Messaging on your iPhone, then your carrier might not support this feature.
Make sure that your device has enough space to receive images and videos.
Make sure that you entered the correct phone number or email address for the contact.

Multimedia Messaging Service is a standard in mobile messaging. MMS messaging is a way to send a message from one mobile to another. MMS can include not just text, but also sound, images and video. It is also possible to send an MMS message from a mobile phone to an email address.

SMS (Short Messaging Service) is limited to 160 Bytes of information. MMS does not have a limit as such

this is not really a problem but understanding should be clear:
Multimedia Messaging Service(MMS)

MMS is a descendant of SMS (Short Messaging Service). MMS extends text messaging to include longer text, graphics, photos, audio clips, video clips, or any combination of the above, within certain size limits.

MMS is frequently used to send photos and videos from camera phones to other MMS phones or email accounts. Most camera phones have MMS, but many non-camera phones have it as well.
if it has a photo, video or audio it will automatically be categorize as MMS
While SMS stands for short message service. SMS is also often referred to as texting, sending text messages or text messaging. The service allows for short text messages to be sent from one cell phone to another cell phone or from the Web to another cell phone. purely text:
just leave a comment if you have additional queries...
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Roger

FOR SMS SENDING CHECK TEXT MESSAGE CENTRE NUMBER IN MESSAGE SETTING OPTION

FOR MMS CHECK YOUR GPRS SETTING WHETHER IT IS ENABLED IN MMS SENDING OPTION

1) go into the menu
2) click messaging
3) click menu in the bottom right hand corner
4) click MMS options - this should bring up a screen with 5 tabs along the bottom of the screen (sending, recieving, profile, compose, about)
SENDING
priority : urgent
expiration : none
delivery time : immediately
message size : 300kb
hide address : unchecked
request delivery report : checked
request read report : unchecked
keep a copy : checked
RECEIVING
home network : auto download
roaming network : auto download
send delivery report : checked
send read report : checked
reject unknown sender : unchecked
advertisement allowed : unchecked
information allowed : checked
PROFILE
contract MMS
COMPOSE
MMS class : moderate imaging
creation mode : free

It is doing this because most carriers only support 140-160 characters per sms message.  Once you are over your carriers limit per message the phone is made to change it from an sms to an mms message.  Naturally when you send media it goes through mms so that is the default symbol for mms on your phone.  MMS supports up to 1000 characters per a message.  So in order to continue using sms you need to keep your messages at 140 characters or less including spaces

try *7370#

master reset the phone and then try again.

The settings are all different, Internet, Wap etc... for MMS you need to call your service provider and ask them to send you the MMS settings, they are sent via a txt msg, when you receive them click Save or Install.
MMS doesn't work between all networks.