This is from Avayas site
A pickup group is a list of phones where each member of the group can answer another member's calls. For example, if you want everyone on one floor of an office to be able to answer calls to any extension on that floor (in case someone is away from their desk), create a pickup group that contains all of the floors extensions. Members of a pickup group should be located in the same area so that they can hear ringing at the other extensions in the group. Note that each extension may belong to only one pickup group.
My Explanation
The end user would simply dial a feature code (which could be programmed as a speed dial) to pickup any ringing phone on their floor. This differs from using *20* + ring group as the end user doesnt need to know/lookup or think about the ring group code (as the other extensions on their floor may not even be in the same ring group) - they just dial a single code to pickup a ringing phone near them.
EG - Floor One in an open plan office has Accounts and Sales which each have their own ring group (100-Sales and 200-Accounts).
After hours people in Accounts wish to pickup calls ringing in sales, but from their location they cannot determine if the phone ringing across the other side of the office is a sale or accounts phone. In this scenario with the existing 3CX functionality, they would have to lookup the ring group number for Sales and then dial *20*+100 to answer the call, but it may not be a sales call and they would then have to look up the Accounts ring group number and redial *20*+200 to answer the call. This is a lot of effort on the users part to answer a call that isn't even for them - so they wont use it.
The other optioin the user has would be to dial *20* and dial in order to pickup the ringing call - but this carries a risk (in a large company) of the user picking up a call that came into the system before the call they are trying to answer, and is ringing in a different part of the building - result = end user picks up a call they dont know how to deal with or dont wish to deal with.
The solution is a pickup group - if all sales and accounts phones are in a single pickup group due to their physical proximity to one another, the after hours accounts user only has to dial the relevant feature code plus the pickup group number to pickup any call ringing
on their floor - this is how a lot of systems do it (Mitel, Avaya, Asterisk, Cisco) and it should be possible in 3CX.
Paul