GS500 Lync PSTN call on AVM CU

Hi,

I had a GS500 with version 5.0.2 registered to Lync 2013 and Lync interoperability license installed. basically the GS able to call  any Lync client and also able to escalate the call to the AV MCU when it add another Lync client to the call.

the problem that if the GS500 is trying the add a PSTN call through Lync "Lync integrate with PSTN GW" to the current call with Lync client, the call is escallated to the AV MCU and both GS500 and Lync client are on the AV MCU but the PSTN call doen't complete.

if the GS500 dial a PSTN call first then it is not able to add any other participants to the call.

any advise what could be the problem?