Your date might remind you of your time constraint (“But I thought you needed to be home at 8…”). People who are dating online actively will appreciate your planning for a bad date and like the fact that you are saying to them “this is a good date”.
If you would rather not let them in on your planning for the worst-date-scenario, just say that you’d prefer to cancel your previous plans and spend your evening with them.
I never went for phone conversations as a stepping stone from internet communication to meeting.
I tried phone conversations early on but had a few bad experiences with “excessive calling” and one case where a girl took it as a personal offense every time I had to get off the phone.
Actually meeting the person you are talking to is one of the most important steps to online dating.