Nokia 1010 can't get back onto Network

Folks,

I removed a SIM card from the above phone to see if it worked on another phone because the other phone wasn't working. I put the SIM card back in but when you ring the number it goes straight to voicemail and I can't ring other numbers because i get an engaged tone. It's on the Vodafone network. The SIM card works fine in terms of making and receiving calls in the other phone i.e the SIM card on the other phone needs replacing. What could be the problem with the Nokia phone. If anyone can post an urgent possible fix it would be great. Thanks