The mystery of bridges that are not opened by VHF request was laid to rest today by sailing up to one and reading the notices thereon. My translation site (https://www.freetranslation.com/en/translate-english-dutch) suggests the following, which is enough to get the gist:
per ship 1x pressures for bridge control
at against shipping wait until bridge is closed
Misbriuk press is a punishable offense!