After deinstalling Public Beta and installing RTM I cannot make any type of phone calls using Communicator, neitzer internal nor external. Error ID:18 is shown which is not documented...
Looks like the link provided is no longer available. Can you give some hints about how you solved your issue about the ID:18? I have the same error while trying to place calls or videoconferences between Office Communicator clients.
Just as I wrote: I had my line URI as tel:947 and got that error. After changing to E.164 format, e.g. +493328455947 it worked. You may have to add/remove the E.164 addition through rules in gatewayor OCS.