Aight I sent a mail to my ISP and here's their reply:
Dear Etisalat Customer,
Thank you for contacting Etisalat Customer Care Center.
Further to your email, we sincerely apologize for the delay. Please note that using VOIP for pc to phone calls is illegal as per the UAE's internet access management policy for providing VOIP services. Hence, all web portals providing VOIP services would be blocked by Etisalat.
They say that using VOIP for PC to phone calls is illegal and that they blocked mumble's VOIP service for that reason, does mumble even have such a feature?
Edit: Lol I just remembered that even Skype is not blocked, seriously wtf is with these people?