If this happens to you, wait a few days (continuing the email conversations) and then ask again.
If on the other hand the date is going excellent, suggest to your date that you extend it.
If you really hit it off on the phone (for example, did you talk for two hours without even realizing it? Regardless of the success of the first phone call, I still think the first date should come rapidly so don’t wait too long to move from phone conversations to a real meeting. Hint or specifically state that you can only be out until a certain time.
Most people will accept this or make a joke in an attempt to fish for more information (“What, have another hot date after ours? You can come up with a reason you can only be out for a set amount of time but most people will not press very hard to find out why.
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.