The registration fees for conferences are always taxable for VAT in the country where the conference is held. Regardless of who provides the service or receives it.
If in the country concerned is no tax exemption, the service provider must charge therefore local VAT (in country of conference).Unless a diversion of tax is required by the local rules of that country. This should be checked for each country
A diversion of taxes does not apply if the participants are not from the country of the event (even if the participant has a tax identification number). With regard to non-EU participants in an EU conference the service provider always has to calculate local VAT about the participation in the conference.
If someone wants to reclaim foreign VAT, then they have to reclaim that from their local tax institution. The same applies e.g. for the VAT you paid at hotels where you are staying abroad.
If a conference is held in Belgium and the access is granted by an entrepreneur established in Belgium, the fees of the congress are regulated under Belgian VAT. If in that case in no exemption for the conference is possible, then the Belgian service provider has to charge Belgian VAT to foreign (and Belgian) participants regardless of whether the customer has a VAT identification number.