Pickup groups mean that, within a set group of people, anyone can pick up calls to a person’s phone from their own desk phone, by dialling **3.
If you are not in the office, then you can't make use of this Pick up group feature, as you will not hear someone else's desk phone ring. When you are in the office, if you are using a desk phone, then you will be able to continue to pick up someone else's desk phone. If you move to Telephony over Teams, then you will not be able to use this feature.