If I have Flight mode on will people know I'm abroad? - 1

I have an Iphone 5s and want to take it on holiday, however, there are certain people I don't want to know I'm abroad.
If I turn Flight mode on would anyone phoning from the UK know I'm abroad?
If flight mode isn't on would they know? & how would they know?

No- to all of these

They couldn't know that. Because when you active the "Plane mode" your iPhone switches off all the wireless modules inside (Cellular, Wi-Fi and Bluetooth chips and antennas) to avoid to release any electromagnetical signals.

If someone from any part of the world calls you while your phone is on "Plane mode" would be like your phone were turned off. Your carrier Can't know if your phone is on Plane mode, out of coverage or turned off.

I don't know what would exactly happen if people calls you from UK to another country. But in some countries phone "dial tone" is different, so people could notice. Example, if you call from UK to Spanish phone you'll notice that the "beep" dial tone sounds different than the UK,

… In UK the dial tone is like: Brrrrrr - Brrrrrrr --------- Brrrrrr - Brrrrrr… But in Spain is like: Beep ---------- Beep
(LOL. I'm feeling So onomatopoeic)

But I dunno how it'd sound when people calls you if you're abroad and your number is British. You'd rather to avoid roaming fees…