Asterisk Install w/ Existing Phone System

I currently have Asterisks set up with FreePBX with two extensions for testing. We have an existing phone system on the same network with a ten port card for VoIP support, which is maxed out. I do not have VoIP service for the Asterisk setup at this point, but I would still like to perform tests. However, the two Asterisk extensions can call any extension from the existing phone system by default.

I would like to be able to test the Asterisk extensions, as well as features such as voicemail, but this does not seem like it is going to be possible. Any thoughts as to why I can hit existing extension from Asterisk extensions, and why I cannot dial Asterisk feature codes?

If your 10-port card is an Analog card, you can buy a Sangoma card that supports 20 ports and install 10 FXO ports and 10 FXS ports. Then install your FreePBX box between your existing phone system and the telco. I have done this successfully with 2 installations.

I appreciate the response John, but I suppose I should have been more specific with my questioning. I do not want the Asterisk server to be able to call extension on the existing phone system automatically. I am setting up Asterisk because we now have two companies in the same location. The existing phone system function well, but I do not want to put anymore of a load on it.

I suppose I should change my question to:

Can the Asterisk server call existing extensions simply because they are on the same network? How might I work around this? Would this be a place for a virtual lan consisting of the the asterisk server and the sip phones, or is there a simpler method if I need to have the existing phone system and the asterisk server on separate networks?