Wait, are you saying multitasking doesn't work AT ALL on a jailbroken iPhone? For any apps?
Check out this thread
https://forums.macrumors.com/threads/982893/
For me at least, it seems that without Backgrounder app, everything else WORKS except Skype.
Tested: Pandora, Wunder Radio, Reeder, Kindle, Settings, Mail, SMS, and iPod.
With Backgrounding app, everything works including Skype. However, there is a problem with this method because Backgrounder kinda make some apps that shouldn't be running on background... well run on background.
An example is that Meebo shouldn't be running on background but only giving me push notification. Without the Backgrounder app, I managed to get Push notification fine and the Meebo app itself is NOT running when checked with SBSettings Processes. With the Backgrounder app, that app is RUNNING on SBSettings Processes... which may drain battery life.
Now, some people have tried to approach it the opposite way and install Backgrounder but DID NOT enable it at launch. Rather, they put exception on each app that they know works with Multitasking and iOS4. This approach is fine except you have to actually put the exception on each apps otherwise they won't run on background at all.
I tried to only add Skype to the exception and yes the background calling works, but it breaks WunderRadio, Pandora, and Reeder multitasking....
I'm not sure what exactly happening but for now I'm giving Skype middle finger and not going to install Backgrounder.