SMS and MMS are often presented as two settings on one feature. They are two different services that happen to share a sender and an inbox on the handset.
What does each actually carry?
SMS carries text inside a small, fixed payload. That constraint is the source of everything distinctive about it: 160 characters in the compact encoding, 70 in the Unicode one, and longer bodies split into segments that the handset reassembles. What is an SMS segment and GSM-7 vs UCS-2 cover the mechanics.
MMS carries media: images, audio, video, and text bodies well beyond the SMS limit. It does not travel inside the SMS payload at all. The message points at content the handset retrieves over data, which is why an MMS can be large and why it behaves differently on a handset with no data connection.
Can any number send MMS?
No, and this is the first thing to check rather than the last.
Support is a property of the number. Bird's numbers overview describes the field directly:
capabilitiesis what the number can actually do:sms,mms,voice, or a combination. A number that listsvoicecan carry calls; one that does not, cannot. Filter a search by capability when you need a specific one, and check the field before you build on a number you already have.
The same guide notes that a number allocated to a workspace "can carry SMS, MMS, or voice depending on what the number supports". So capability is decided when you acquire the number, and a plan that assumes MMS should filter for it at purchase. The numbers overview covers checking the field on a number you already hold.
Why is MMS coverage so much narrower?
Because more has to line up for it to work.
An SMS reaches essentially any mobile handset anywhere, because the service is universal and ancient. MMS depends on the recipient's network supporting it, the route carrying it, the handset retrieving the media, and a data connection existing at the moment it tries. Any of those failing produces a worse outcome than a plain text message would have.
Availability also varies sharply by country in a way SMS does not, which makes MMS a regional decision rather than a global capability. What a given destination carries is published per country on the SMS destinations page rather than stated here, since it changes.
When is MMS worth it?
When the media is the message, and you can accept narrower reach.
Good cases are ones where an image carries information text cannot: a delivery photo, a boarding pass, a visual receipt. Weak cases are ones where the image is decoration on a message that would have worked as text, since you are trading reach and cost for presentation.
The decision that gets skipped is fallback. If a recipient cannot receive MMS, something has to happen instead, and nothing decides that for you. The workable pattern is to send text with a link to the media, which reaches everyone, renders on any handset, and gives you a click signal MMS would not. Reserve MMS for the cases where the media genuinely must arrive in the message rather than behind it.
One consequence of the link approach is worth knowing before you build it: carriers treat public link shorteners with suspicion, so the link should be on a domain of yours. Why do carriers block public link shorteners covers it.