Can you swap a sim from an iphone 4 to an android?

So my boyfriend has had a iPhone 4s for little under 2 years and he's just got a HTC One i gave him, When we swapped his sim card from the iPhone to the HTC we got a signal and he can text and ring people but can't activate his 3G network, it keeps saying its disconnected… What is the problem?

Added (1). The thing is the iphone was vodafone mine was unlocked

The problem is that on his contract he is paying for XMB/GB on data for the iPhone. The can be change by logging onto his account and changing it from iPhone to Smart Phone

An iphone is a smart phone. If both phones are branded by the same company and the sims are same shape it will work just fine.