Unfortunately, I can only talk for Airtel. For Airtel, you do not need to roam, it automatically roam itself. As soon as we landed at Accra, I received a message welcoming me to Ghana from Airtel. My phone too showed the corrrect time of Accra.
The call rate within Accra was very cheap. I made a local call for 3mins. 12seconds and was chrged N26.00!!!!!! but any call from Naija na gbese if you pick o!
Calling naija too was relatively cheap. You may need to ask MTN via their customer services centre if you need permission to roam. But me I didn't collect permission to roam, omo I roam tire!!! GBAM GBAM GBOSA ;D ;D ;D