Had a recent customer where conferences initiated with outside PSTN participants would fail. Internally conferencing between Lync users worked fine.
The customer is using a cisco ISR gateway with 2 PRI's to the carrier. There is also 4 digit dialing set up between Cisco UCM 4.x and Lync. Yes you heard right a windows version of Cisco UCM. Additionally, ingress and egress calls from the PSTN work fine.
So after confirming nothing was misconfigured on the Conferencing or Client Policies it was time to bust out ocslogging utility and sniff through snooper logs.
Decided to log SIP, S4 and MCUInfra and received couple of SIP 500 unexpected internal errors
ms-diagnostics: 4099;reason="Internal Error: Failed to validate GRUU contacts";HRESULT="0xC3EE79F2(ES_E_WRONG_MCU_TYPE_IN_GRUU)";gruu="sip:email@example.com;gruu;opaque=app:conf:audio-video:id:1HBHY447";source="Lync.acme.com"
The issue turned out to be the certificate assigned to the Front End server. It was an external certificate as an internal PKI was not available at the time of install. The issue was remedied once an internal certificate was requested and assigned from the newly installed Certificate server. There was a name mismatch on the Public cert.