Yes, that must be it. If I answer when the screen comes up as unknown it is always a telemarketer of sorts.
You could install a 3rd party app to show you if it's spam or not. Alternatively, if you own an Android device, you can search Caller ID and spam protection in your settings and turn it on. Your device will tell you if the caller ID has been reported as a Telemarketer or not.
Hey @fionaS. You might also want to take a look at our article on nuisance calls here. I'd suggest putting your number on the Do Not Call Register to prevent any legitimate telemarketing calls from being made to your number. Please let us know if you have any further questions