To switch from one handset to another on the Gigaset Comfort 500 A Trio, you can use the call transfer function. Here's how to do it:

1. Make sure all handsets are registered to the same base station.
2. During a call, press the transfer key (usually indicated in the manual).
3. Select the handset to which you want to transfer the call.
4. Confirm the transfer to transfer the call to the other handset.

If you need more details, consult the user manual for specific instructions.