I think you can using such devices like Analog Telephone Adapters.
For example:
ZyXEL P2002 VoIP Analog Telephone Adapter, Planet VIP-156 Adapter (SIP Analogue Telephony Adapter (ATA) - 2*RJ45) and others.
Try to search "Analog Telephone Adapter" in Google. There are many kinds of them.
I have an old LinkSys PAP2 adapter from Vonage. Can this be reconfigured to connect my analog phone to the 3CX server?
Here is the link for the product if that helps:
http://www.linksys.com/servlet/Satellite?c=L_Product_C2&childpagename=US%2FLayout&cid=1117044308483&pagename=Linksys%2FCommon%2FVisitorWrapper&lid=0848387006B01
I have configured 4 extentions using 2 PAP2 ATA's.And using a SPA3000 to have a PSTN line hooked up to the PABX.
You configure the PAP2's to point to you 3CX instead of your VISP.
EG: Proxy:192.168.10.10 (this is the ip of your 3CX pc.)
Outbound Proxy: 192.168.10.10
USER ID: 801 (this is your selected extension)
Password: 801 (I always use the extension number,easier)
Auth ID : 801
Use Auth ID : set to yes.
This works for me....
PS also make sure the PAP2 is not locked to Vonage....If it is it needs to be unlocked.