Install fring and make sure you have an Internet connection on your mobile / smartphone (through UMTS, WiFi, HSDPA, GPRS...).

Now start fring.
Then goto Menu, Settings, Configure Services, select SIP, scroll down to "Other".

There fill out just these 3 things:

User: 7xxxxx (your 6 digits GismoTel account number)
Password: password (your password from your GismoTel SIP account)
Proxy address: sip.gismotel.com

That's all.

Now start calling with fring and save on International GSM roaming.