After a call is completed, you are taken out of the phone service for a short while, to be able to finish your notes from the call. During this time, you are not receptive to new calls. The number of seconds depends on whether you answer the call in the client or via a forwarding, and is a standard time for 5-10 seconds.

You can change the wrap up time under Settings, which you'll find by clicking your profile picture in the desktop client, and under Phone.

During the Wrap up time, you have the option to extend it by clicking on +15s, +30s or +45s that appears when you end a call.