You need to be wary of the terminology - many mobile apps will talk about "VOIP" and use it to refer to a bundled voip service and a bespoke app integrated with it. These are/were popular for discount international calls etc.
That is not the same beast as a generic voip client that understands generic SIP accounts. Some Android phones have built in "Internet calling" or "SIP Calling" capability which can use a voip service to make and receive calls.
Failing that, look at apps like linphone or zoiper...