VoIP has historically been a second class citizen in Riot, but we're trying to fix that right now. The implementation in RiotX is in mid flight up at https://github.com/vector-im/riotX-android/compare/feature/v... and Riot Android will be killed off shortly in favour of RiotX.
RiotX Android doesn't support calls at all according to the Play store page, while the old Riot Android client doesn't support cross-signing, search doesn't work, and it overall feels unpolished. It's still very much beta software.