Initially it was within the same network, I believe Orange (previously Hutchinsons, then merged with EE, who themselves known as T-Mobile) was the first to offe

Initially it was within the same network, I believe Orange (previously Hutchinsons, then merged with EE, who themselves known as T-Mobile) was the first to offer a consumer service, called Orange Messaging. As an aside they also offered a second line on the same phone,meaning you had 2 number an option not offered these days alas. But nice trick was to set a foreign SMSC, this not only enabled you to send SMS to other UK networks, but also didn't charge you. Circa 1998 era.