DNS settings are important and you need to understand how your organizations Skype for Business communication flow works.
First important understanding:
If you run a hybrid installation, your Office 365 Tenant with Skype for Business Online is seen from your On-Premise installation as a federated organization.
Therefore the following DNS records must also be resolvable from your internal DNS infrastructure (Edge Server).
Depending on how DNS is configured in your organization, you may need to add these records to the internal hosted DNS zone for the corresponding SIP domain(s) to provide internal DNS resolution to these records. (see illustration below table)
DNS RECORD RECORD TYPE WHERE IT SHOULD RESOLVE TO PORT sip.YourDom.com A Public IP of Access Edge n/a _sip._tls.YourDom.com SRV External on-premises Access Edge Interface (sip. YourDom.com) 443 _sipfederationtls._tcp.
YourDom.com SRV External on-premises Access Edge Interface (sip.…