Swytch is an iOS and Android app that enables you to have multiple 'burner' numbers on your existing device - without needing separate SIM cards. It's great if you want to have a seperate number for work use, but you want to use your personal device rather than having a second phone.
You can have up to five numbers nested within the Swytch app and keep them as long as you want - they cost £4.99 per month each. However, there is an additional cost if you want to make calls on your Swytch numbers of 6p per minute. Calls between Swytch numbers are always free.
Swytch will tell you which number an incoming call is coming through on and even give it a different ringtone - so you know whether you need to pick up or not.