Where do I enter customer's credit card info, to create a subscription for them over the phone?

You can create auto-charging subscriptions for customers that have a credit card already linked to their account. This is so because credit card processors are very strict when it comes to automatically charging the customers' credit cards, which means that the customer has to go through the process of placing an order in your shop, where they agree with the subscription policy and they insert their credit card info. After that, Shopify can request the authorization from the credit card processor to automatically charge their credit card.
This is the most secure and standard process in the industry and we can't bypass it.

Entering the credit card details manually is not yet supported in our app. This is due to technical and security limitations.
The best workaround here is to place an order in the store in the customer's name if you have the permission from them to do so. This way, you can start a subscription for them, but only if you have explicit permission from them.

The other workaround here is to create a quick checkout link in Seal Subscriptions > Subscription rules > Quick Checkout Wizard and then send this link to the customer, so they can complete the initial subscription order and pay for it on their own.