Multiple Prefix


I am new to FreePBX and in need of some help. we have numbers in the same area code; multiple prefixes ie XXX-855-4XXX or XXX-399-9XXX.
How do I create these in the system? I have created the sip file using the full number however I would like to use just the extension (all extensions are new and do not match existing)
Pointing me to an appropriate area or even answering would be helpful!
Phones are Cisco 7940.