If you need to post your phone number online for a dating site or if you are selling some items online then you should always use a disposable number.
A good app for this purpose is "Burner". Just search for "Burner app on iTunes
or Google Play
After the registration they give a free number for1 week. After thsi week you have the option to choose a number of inexpensive plan.
This service of google is good for texting and calling. You just need a normal account on Google and then you are able to receive a fresh phone number free of charge. The only restriction is that you have to be an US citizen and you have to enter your US number for verification.
A phone number to receive SMS anonymously
Sometimes you need a phone number just for 1 or 2 text messages you want to receive for a verification process. For this purpose I can recommend to use websites that offer free phone numbers just to receive SMS messages. The good thing is that you do not have to register an account or anything like that. Visit their website, choose one of the provided phone numbers and enter the chosen phone number in your app where you need the verification code. Wait some seconds for the arrival of the text message and enter the code in the app. This method works faster then to register a new phone number on Burner or Google Voice. Here are some websites I can recommend: