T48G UC Edition Message (Voicemail)

[abeggled]

I'm unable to dial in my voicemail from my t48g. Not by clicking the Message button, nor by dialing "99" my voicemail number.
With other phones or the lync client everything is working as expected.

Exchange UM is logging these events:
The Microsoft Exchange Unified Messaging Call Router service rejected the call for the following reason: 15605;source="SRV013.xxxx.xx.xxxxx.net";reason="Invalid SIP header." Microsoft.Exchange.UM.UMCore.InvalidSIPHeaderException: The Microsoft Exchange Unified Messaging service on the Mailbox server received "INVITE" request with an invalid SIP header "USER-AGENT" with value "".
at Microsoft.Exchange.UM.UcmaPlatform.UcmaSignalingHeader.FromSignalingHeader(Signa​lingHeader h, String context)
at Microsoft.Exchange.UM.UcmaPlatform.UcmaCallInfo.<get_RemoteHeaders>b__0(SignalingHeader x)
at Microsoft.Exchange.UM.UMCommon.ExtensionMethods.ConvertAll[TInput,TOutput](IEnumerable`1 inputCollection, Converter`2 converter)
at Microsoft.Exchange.UM.UcmaPlatform.UcmaCallInfo.get_RemoteHeaders()
at Microsoft.Exchange.UM.UMCore.CafeRoutingContext..ctor(PlatformCallInfo callInfo, UMADSettings config)
at Microsoft.Exchange.UM.UcmaPlatform.UcmaCallRouterPlatform.TryHandleIncomingCall(​CallReceivedEventArgs`1 args, Exception& error, Boolean& isDiagnosticCall)
at Microsoft.Exchange.UM.UcmaPlatform.UcmaUtils.<>c__DisplayClassf.<ProcessPlatformRequestAndReportErrors>b__d()