Make sure you have a jailbroken devices, preferably an untethered jailbreak.
Watch This Video First:
Spire (Siri on Pre A5 Devices)
If the following video didn't help, please read these step by step instructions.
1st - Open Cydia
2nd - Open the Search Tab
3rd - Search for "Spire"
4th - Install Spire (100MB, longer download time)
5th - Open Setting Application
6th - Open the General Tab in Settings
7th - Open the Siri Tab with in General and Enable Siri
8th - Open the Spire Tab within Settings
9th - Insert a Proxy Host (Purchased or use one of the Free Proxies below.)
Spire Proxy Host List:
CONFIRMED PROXY HOSTS
CONFIRMED PROXY HOSTS
Name: iModZone
Cost: $20/year
Website: http://imzdl.com/siri/
Verified: Yes
Name: iCrackr
Cost: $20/year
Website: http://icrackr.com/
Verified: Yes
Name: The Three Little Pigs
Cost: Free
Website: http://paradox-productions.net/
Host URL: https://paradox-productions.net/
Certificate: http://paradox-productions.net/files/certs/cerftificate.pem
Cost: $20/year
Website: http://imzdl.com/siri/
Verified: Yes
Name: iCrackr
Cost: $20/year
Website: http://icrackr.com/
Verified: Yes
Name: The Three Little Pigs
Cost: Free
Website: http://paradox-productions.net/
Host URL: https://paradox-productions.net/
Certificate: http://paradox-productions.net/files/certs/cerftificate.pem
After you insert your Proxy Host, Siri should be fully functional. You may have to Reboot your iDevice, and possibly re-enable Siri in the Settings App.