Cecilia,
that's just what I feared. Quite some PA mixers do have a rather minimalistic, non-standard-conforming view of what is phantom power, and what isn't.
Obviously, I could recommend our N48i-2 or battery powered BS48's, but any other decently manufactured external 48 volt phantom power box should do.
Please check beforehand that: 1. the phantom power is blocked off on the leads to the mixer >> otherwise there might be a conflict between the 48 and the 15 volts >> failure. 2. the external phantom power box does block off and can handle the 15 volts coming from the mixer, without damaging the box. Or, keep the 15 volts switched off.
Technically speaking, it's a matter of correctly designed blocking-off capacitors.
Best regards, Martin Schneider Neumann Mic. Development