Most probably this won’t be important for those who are doing migration to Office 365 services consistently and in full scale. Because DNS records for all services are mentioned when first configuring new Office 365 tenant and creating a special DNS record to prove your ownership of a domain. But as we only needed to assign Office ProPlus licenses at first we have skipped creating additional records. Later, after receiving Skype for Business (SfB) licenses, we have decided to test communication with external SfB and regular Skype contacts. It failed. Although it was enabled in the SfB admin center. But it still was giving an error that some policy is blocking such communication (not a very helpful error message btw). It turned out hat we needed a few special DNS records on our domain to make it work. Examples can be found at Office 365 Admin Center > Settings > Domains > domain.tld.
|Skype for Business (CNAME)|
|Type||Priority||Hostname||Points to address or value||TTL|
|Skype for Business (SRV)|
After these records have been added to our DNS system we were able to add external SfB and regular Skype contacts, send messages and make video calls with them.