Can I use a Telecom iPhone with a Vodafone sim card?

Hi I'm from new zealand! So please make the answer relevant to me

Yes if its unlocked

If the phone is locked to a Telecom sim card, then not without unlocking the phone first. If the phone is unlocked, then yes.

At the minimum, you'll have 2G service. Depending on the phone model and the bands those carriers provide 3G and 4G LTE service at, you may or may not have higher mobile broadband speeds.