If your local phone operator has a roaming agreement with an operator in Malawi, your GSM mobile phone should roam fine here. There is pretty good coverage across the country. If you want locals to call you you will need to follow the insructions below, otherwise the local will not be able to call you unless they have an international call setting, most unlikely unless its a business user, as the call is routed all the way via your home country,  this ends up being very expensive. Bear in mind that you will may well be charged an international rate for any incomming calls from your home country, the caller only paying their normal rate. Check with your service supplier, if you are a contract user, they may well have a special rate where you pay a small monthly fee, and get a good reduction on the international charge for incomming calls. You can always cancel this when you get home.

If you have a GSM mobile phone, check with your provider to make sure the phone is "unlocked" to used with other providers, then when in Malawi you can buy a SIM card for your phone for $10 or less, and use on a pay as you go basis. That way you'll have an African telephone number, but then you can make calls within the country pretty reasonably, and anyone calling from abroad will pay for all of the call.

Bear in mind that there is a standing charge for the line, so even if you dont use the local sim card, it will run out eventually.

The two competing mobile phone companies are Airtel, and Telekom, both are good.