There's a few mobile phone holders available which can be mounted to the handlebars. Topeak is one brand that comes to mind. You could then get a bluetooth headset and away you go.
I am sure you can adapt a cheap bar mounted bottle cage. Put the phone is the empty bottle with a bit of foam. I use old screw top plastic medicine jars for tools, etc.. they fit the bottle cages perfectly.
Believe it or not, some of us have bosses who expect instant access via cell phones and Blackberry all weekend. Retirement is the fix of course....worked for me