Someone's using my iMessage?

Been using this number since April and I believe this number used to belong to someone else since I've been receiving phone calls from different numbers asking to speak to a certain person. Last night someone tried to register my number on whatsapp I kept receiving messages with an activation code but I ignored. This morning I realized I can no longer receive iMessages on my phone it says that my account has been signed out of iMessage and when I try to activate it again it says 'we're not able to contact this number.' This is very inconvenient cuz it's happening on my work phone and I need my phone and my iMessage to be available all the time. Anyobody knows how to fix this?

Updates
+1 y
They're not using my apple ID they're using my phone number.
Someone's using my iMessage?
Post Opinion