Samsung Galaxy smartphone with mobily sim and Zain subscription!

I just bought a Samsung Galaxy Smartphone.  I put a Mobily sim card in it (which my company gave me in a nokia phone, when I first arrived three weeks ago).  And I went to a Zain store and paid 75SR for one month and 200 minutes of calls.  However, I still can't use the phone - I can't make any calls.  When I try to, a Mobily message says I have insufficient credit, but I have a subscription with Zain.  I don't know what to do.  Could someone please advise me?  Many thanks.

One question, why did you pay to Zain when you have Mobily sim?