This site, serviceshub.microsoft.com/supportforbusiness, should be where to go for opening a new support case.
When you go to this site, you'll be asked to login with your work email but this should be the email you've used in the past to log into PartnerSource/CustomerSource.
Once you begin entering the details of your support case, the system will validate that you have an active support plan. If you know you have a support plan, check the email you used to log in with, to make sure it is associated with the business' account.
You can also login to the CustomerSource Center here >> mbs2.microsoft.com/.../default.aspx >> here you can click 'Manage Accounts' and verify information is setup correctly. On this page, you can also 'Link your Accounts' and follow the prompts as needed.
If none of this work, email the itmbssup@microsoft.com team, as they handle all the internal sites, access and troubleshooting.
The 888-477-7877 number also works for opening a support case.
Thanks
