Apple

I'm a new iPhone user and texted my sister at her personal cell number. Now, her personal number is the only number I've ever had for her, and it's been the same number for 18 years. But she replied, "Hey, why are you messaging my work number??" Confused, I asked, "Oh my bad. The only number I have for you is XXX-XXX-XXXX. What's your personal number?" She said, "No, that's my personal number. YYY-YYY-YYYY is my work line, and that's what you texted." But my sister has never given me her work number, and we've verified that the only number I had was in fact her personal number.

Could anyone explain how on earth my text to her reached her work number that I've never had? I'm assuming these two numbers must be linked somewhere, but would that be on her end? My sister's an Android user, and when I was an Android user, this never happened, so maybe this is an iPhone thing? Either way, we'd like to prevent this happening again, so I'd love input.