We would like to use the Skype service (or other VoIP services) in our
company through our regular PBX (in order to maintain some control and
not have to invest
in new phone hardware). We want the incoming IP calls to be received
by the regular PBX welcome message and routed acoording to the dialed
extension or directed to the receptionist as it is done now.
Are there any gateways/hardware that allow such conection or any other
solutions ?, is there any software to keep some type of traffic log ?,
will there be any security issues ?

Thanks

The link below is for a device that creates a phone line that can be
connected to a phone system through an unused C.O. port. It is just
like the circut that would be provided by your local phone company.
The reporting option should already be part of your PBX.
http://us.accessories.skype.com/direct/skypeusa/itemdetl.jsp?prod=2916

How many office phones and phone lines do you have hooked up to the current system?

What you are describing is very commonly done and any Cisco Voice
Certified Reseller will know exactly what you are wanting.

So yes, what you need is a VOIP Gateway, here are the basics of this...  

1) You need to check and see if your PBX has an available analog FXO
port on it first.

2) If it does, you need a VOIP gateway. (Linksys PAP2 or a Cisco
Multiservice Router(there are many models))

3) All you have to do then is plug the VOIP Gateway into this FXO port
using the standard RJ-11 connector. Some PBX's will present the FXO
ports on an RJ-66 block or similar connector, but electrically it is
identical but just a little trickier to connect.

5) Then the rest is up to the person who programs your PBX which
should be very easy to do actually.

 
Also, all of Cisco Systems Inc. multiservice routers have the ability
to act as a voice gateway, and they also offer all types of physical
connections that can be plugged into the gateway to make it easier to
interface your PBX.

If you want to do it yourself, go with the Linksys product instead,
but you won't get the level of support and commitment to make it work
as you will if you go with a Cisco certified reseller.

Good luck, jb