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When choosing the map service 'OpenStreetMap...' or 'Bing Maps', while a (local/national) coordinate system is already active, Visum displays a dialog with the following text:

'You have chosen a map service that supports map display per tile.
Maps can only be displayed per tile if Visum uses the same projection as the map service.
Otherwise, the map can only be displayed after all tiles have been downloaded and transformed.
Do you really want to change the projection and recalculate all network coordinates?'
<Adjust projection> <Retain projection>

What should I do?

This dialog points to the fact, that the chosen map service uses a different coordinate system than currently active.
'Retain projection' will cause Visum to transform and resample the downloaded map tiles. This might degrade image sharpness.
For most networks, using local or national coordinate systems like UTM, 'Retain projection' will be the best choice.
When choosing 'Adjust projection', the map tiles will be displayed as downloaded, and the network will be transformed. This is reversible.

The map services 'OpenStreetMap...' or 'Bing Maps' use the coordinate system 'Sphere Mercator'.