I'd say email them but don't ask for a rec in the email. Reintroduce yourself to them (let them know what classes you were in, when, and how much you enjoyed them), and tell them you want to discuss something with them (or ask them a favor) and ask when a good time to call would be. When you call, update them on what you've been doing since UG and then ask them for a rec. Make sure you give them an out, don't make them feel uncomfortable. So acknowledge that you know they're very busy and they might not have time and you would understand. That way if the prof feels uncomfortable writing you rec/doesn't really remember you, they'll have an easy way of saying no instead of writing you an unenthusiastic rec that will end up working against you.