During a phone call, how do I press numbers for menu selections?

While your ongoing call is still active remove the phone from your ear, wait for the screen to light up again and tap on the “Hide” text down at the bottom right of the screen, which then changes to give you the following options: Mute, Keypad, Speaker, Add Call, FaceTime, Contacts. Tap on the relevant icon, in your case Keypad. During all this your call remains connected. Hope this helps you! Regards, Ian.

Switch to the keypad.

You're very welcome, Marie, glad to have helped you! By the way, it wasn't self-evident to me either, when I got my first iPhone! Ian.