I am not sure if this is possible, bearing in mind that your mobile users whilst out in the big bad world will be using the mobile provider's network to route calls - not your own. (unless you network is insanely big).
It is possible to control the cost of mobile calls when in the office however, there are products available such as installing your own private gsm base station and having the mobiles connect to this whilst on site.
There may be a cheeky workaround actually. In the UK, one of our providers (o2) offers a family bolt-on that offers free calls between configured numbers (that are o2 only). What if you installed a gsm gateway and integrated it with CUCM and Unity. Add this central gsm number to your users' bolt-on service and they can call it for free (in theory). Then configure the call routing from this gsm gateway to route to Unity and set up an auto attendant to route calls or dial by extension.
It may just work, but o2 would not like it for sure.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...