BT's system automatically converts a text which is sent to a landline number into speech (or sometimes, depending upon the content of the text, to a rather vague approximation of speech!).
It's likely that you'll have heard a female voice reading the joke to you (as that's the default setting). However, if the person who sent the text thought that a man's voice would be better, he/she could have typed *M# at the start of the message to achieve that. He/she could have also got the voice to blow kisses to you. Adding xx at the end of the message makes the voice say 'kiss kiss' but typing xxxx makes it say 'loads of kisses'.
Chris