In the Teamup app, you can call a phone number directly from event details. The phone number can be entered into any text field (Description, a custom text field, or a renamed Who or Where field). See more on the event editor.
Here are the specific instructions for iOS and Android.
iOS: Tap the number, then tap the pop-up to call the number.
Android: Long press the number, then tap the pop-up to call the number.