Hi Al,

We use Lync as a PBX. Calls come in and go to a response group. From there, they will be transferred to users in the organisation, all in same internal network.

We get a lot of errors saying : SIP/2.o 488 Not Acceptable Here. 

After one or two tries the calls goes well.

Please check if there are any errors on Mediation Server.

Please check the configurations for mediation Server in Topology are correct.

Check the required ports for mediation are opened.

http://technet.microsoft.com/en-us/library/gg398833.aspx

Check the ports for IP/PSTN gateway are opened correctly.

http://technet.microsoft.com/en-us/library/gg425945(v=ocs.14).aspx