Hi Ajaz

were you able to use the "activation-key" command on the ASA ? this is the way we normally upgrade any feature licenses.. not sure about SIP though.. With your PAK key, you can obtain the license key from the URL www.cisco.com/go/license.. once you receive your activation key, you can use this:

hostname(config)# activation-key xxxxxxx

hostname(config)# reload

if you have an ASA failover, you might need to do the same procedure to the second firewall, but you might need to first disable failover, and re-enable it after upgrading the license.

Hope this helps.. all the best

Raj