This is a great article on why not to hire a cold calling SEO company.
I would go a step further and not even take their call. Nothing is more annoying that when one of my clients calls me after being ear banged by a moron about how bad their site is. Then I have to explain why cloaking or link farms will get them banned in Google. Even worse is when someone sells hard that they have backend access to "Inktomi" to change the rank of the site. Seriously? Inktomi has been gone for years!
Work with someone you trust and build a relationship with for SEO services. And make sure you keep ongoing communication happening to ensure the long term success of the relationship.