Check SRV records

Open cmd

Type: nslookup

Type: set type=all

Type the SRV record to list its content

For OCS 2007 R2 External

Type: _sip._tls.domain.com

Usually points to Access EDGE FQDN on port 443

Type: _sipfederationtls._tcp.domain.com

Usually points to Access EDGE FQDN on port 5061 Internal

Type: _sipinternaltls._tcp.domain.com

Usually points to Pool name with correct sip domain on port 5061

For Exchange 2007/2010 External autodiscover

Type: _autodiscover._tcp.domain.com

Usually points to owa FQDN listener with NTLM negotiate on port 443