Skype and 3G

[Mdaf]

Hello, I've noticed the following, Skype is officially blocked in our network but if you tried to access it through USB modem via a laptop it will be accessed being able to make calls, same SIM same cell same each and everything but instead if you use the SIM in mobile, nexus4, sony xperia z, ********.... It won't access Skype!!!
take the SIM and put it in another USB wifi modem same laptop then again you can't make Skype calls....
please any ideas is it related to the way that modems phones request the service being low priority or high....
all have same access point defined.
thanx for you help.