@NootyBoy: You could try heading to Settings > Notifications > Do not disturb, and activate this feature. Now, under 'Allowed during Do not disturb', select Calls and messages > Add contacts, and add your preferred contacts from here. If you're away on holiday, then maybe just include your emergency contacts, or certain friends and family members that you want to stay connected with.
Otherwise, head to the Phone app > Tap the 3 dots in the top right > Settings > Quick decline messages, and create a bespoke message that you can send that informs your colleagues that you're unavailable to take their call. Here, you could ask them to email you, or pass on the details of someone who may be covering for you in your absence. The same could also be achieved with a bespoke voicemail message.
