This function is easy - you just use softphone like expresstalk.Pickup the line ,hit transfer and put in the number in softphone.
Number can be another extension or any outside number. It will work and it does work, I've done it myself.(you just need 2 channels for your sip)
If you have specific problem with set up i can help you.
As far as Axon pbx I can tell you this: I spent 2 months trying to set it up and it was furiously frustrating.Equally hopeless were my requests for help from sip providers.
Everybody protects their "expertise", releasing only small bits of information to make you enough hungry to reach for your pocket.
But now, without much help and being telephony illiterate I have a telephone system comprising of Axon pbx, expressTalk softphone and Automatic call attendant from Axon.
May have cost me around $350 or 400 but it works.
I have 1 tel. number (in US), 1 sip number and 2 channels.
I spend ~ 25 to 30 dollars monthly and I have 5 people connected to one company number, 2 are in US and 3 are in 3 different countries in Europe, plus myself.
We are on 24/7 call service and so far (4 months now) have not had any problems.And it is cheap. I have purchased really good quality voip phone now and I can hear my clients
chairs squeaking as we talk so clear is the reception.
You will hit some potholes on the way but if you have small business and want to be in control of your phone system it is (I think) the way to go.
I am far from recommending Axon and making sales pitches for them; I think the way the have technical support setup and the whole concept of promoting their system is naive.
Unfortunately, the digits behind software are ok and the principal involved is sound. As long as it works I will keep it. GLuck