There are only 2 types of offers and both come from one provide ( Q-Tel ):
1) pre paid which you can get from any Q-Tel showroom, your air time is related to how much you re-fill your account, the refil cards can be baught from nearly any shop.
2) Monthly paid, this can be done from the main Q-Tel offices [ in al Sadd - carrefour ..], you need to get a letter from your sponser and fill in a certain type of contract, then you pay them monthly either at one of thier outlets or on-line, in this case you have an un limited air line.
Hope this will shed some light to hat you want to know.
best regards
Note: a few offers are avaliable on the pre paid card that give you with it a mobile phone.
Best to buy one of your choice from any where in Doha, though carrefour do provide the cheepest ( in general )