First I'd recommend going through the below checklist and then try again:
1. The extension ringing shows its presence and its calls to group members.
"Extensions >> Edit >> Rights"
2. The extension picking up the call (performing the *20*) can see presence and calls of the ringing extension "Extensions >> Edit >> Rights"
3. The extension picking up the call has the "Perform Operations" right enabled.
"Extensions >> Edit >> Rights"
4. Both involved extensions are in the same group.
5. Go into "Settings >> Dial Codes" and make sure that *20* is actually set as the Pickup Dial code and that there are no overlapping Dial Codes (Other Dial Codes set as *20* or something similar like *2, etc)
If even after doing the above you still get the same, go into "Settings >> General" and disable the following option if enabled and save:
Let us know what happens