How can enterprise customers receive enterprise level support with direct contact?
For all users (Community and Commercial editions), you can find support on this community forum.
If you want a direct contact and specific support, and if you have subscribed to a commercial (€) edition (Developer, Enterprise or Data Center), you should talk to your Sales rep, to add technical support on top of your subscription.
Our support offering is described here: https://www.sonarsource.com/support/
PS: Support is included in the subscription if you have more than 30M LOC, or if you are running a Data Center Edition instance.