Some mobile operators block VoIP data traffic, to avoid money loss from plain phone calls.
Depending on mobile operator, VoIP is blocked only on a contractual basis, while others applies actual technical countermeasures to degrade data quality. 
Generally, VoIP blocking is applied only on cheap data plans, while it is allowed on medium/high data plan.
Due to constant and continuos change in data plans, it is almost impossible to maintain an updated list, so just call your mobile operator help desk to gather information
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