This is something that is completely.... foreign... to me. But let's take a stab. When you are abroad, are you using your existing phone number (where are you from anyway?) or swapping your SIM card out for one local to the country you are in? Some more details would also be helpful. Such as... are the calls you are failing to receive all from your country of origin, or local to where you are traveling? Who is your carrier?
Since you can place calls fine, I assume you must have dial assist turned on and properly using the country prefix to make calls back to the UK.
Since your phone actually rings when someone is calling you, that suggests to me that you can receive them fine even though you hear nothing when answering. That really suggests a carrier issue to me. You may be roaming in an area that your carrier doesn't work well with. I'm googling around and can't find any potential solution involving something wrong with your iPhone. Unless someone else chimes in with more qualified advice, I think you are best running this by your carrier. Otherwise, a good solution would be to use a local SIM card while traveling to ensure you can connect without issues locally. I found a guide that seems to be well thought out exploring various alternatives and costs.