I'd say most likely an accidental button press?
Alternatively, could it be a prank? It's not difficult, if you have the right equipment, to spoof the source number on either mobile or landline networks, or to spoof the source number on text messages. An internet search may even yield websites that will do so on your behalf, but I'm pretty sure it's illegal in the UK and so I'm not posting any links.
I strongly suspect that's the simple mechanism that the newspapers used to 'hack' voicemail; nothing clever at all really, they simply spoof the calling number to match the handset's own number, whereupon voicemail can be accessed without a PIN. But I suspect the networks may have tried to close that loophole by now.