Cards to add call credit to your mobile phone are available in petrol stations and supermarkets. You can also add credit to your account using the machines provided by Etisalat in large shopping centres. The SIM card is valid for one year only, after which you will need to renew it if you want to continue using the same number.
The Wasel plan also provides international roaming, so you will be able to use your mobile phone outside Dubai.
More information is available on the Etisalat website, in the Travellers > Visitors section.