If you switched away from an iPhone, you may have issues receiving SMS sent by someone from an iOS device to your number, as those may still be sent to your iMessage account. The solution for this was to switch off the iMessage feature on your old iPhone before you switched but if you no longer have the phone with you, Apple has now released a tool that will let you deregister your number from iMessage.
This online tool allows you to enter your mobile number, which then sends you a code. After receiving the code on your phone, enter it into the website and Apple will remove your number from its database.
Most people are unlikely to face this issue but in case you do there is now a solution for it.