If you can't send an MMS you might have the wrong Access Point Name (APN) settings. They should be set automatically, but occasionally you'll need to pop them in manually. It's a doddle!

On iPhones and other Apple devices, no longer work on iPhone devices.

On Android devices, they can usually be found in the Settings app under Mobile Networks/Networks, Mobile Network, then Access Point Names/APNs. Make sure to tap the new button (it might be hidden in a menu behind three dots).