We are having some weird problems routing calls on an Asterisk box where the only real handsets are Siemens S450's. The screenshot shows the way that Asterisk is configured extensions wise. There are 4 physical handsets attached to the same base station - hence the identical IP. Each of the 4 handsets has been allocated its own extension 5001-5004. Ignore ext 5000 it is irrelevant to the problem. ext 5002 is configured as the Operator.
We have a basic IVR and the intention is that if no extension is selected the call will go to 5002. That works fine on the first call in to the PBX. However any subsequent call hits the IVR greeting but if no extension is selected instead of going to 5002's voicemail (as the Operator is busy with the first call) we get a busy tone which is presumably being generated by the Siemens base station.
The problem is compounded as the client really wants all extensions to ring on an incoming call and only if it is unanswered should it go to the operator voicemail. With the IVT set to a timeout to a ring group containing all numbers with an outcome of *5002 doesn't seem to work - only 5002 rings. You can set the Siemens handsets to answer calls for more than one extension but that doesn't get round the second inbound call getting an engaged tone.