tar zxsvf asterisk-18-current.tar.gz
# chose what yo want include app_macro and sound files
It looks like Debian is going to skip over Asterisk 18. Asterisk 20 is packaged in sid. So for now you can uninstall the asterisk packaging and install asterisk from source as @dicko said. Maybe later version 20 will be available in backports. I like to stick with the packaged versions when possible, but unfortunately here the package maintainers missed an opportunity to have Asterisk 18 in bullseye.