i think its not nationality issue here but thinking negatively, sometimes a caller may ask your nationality to add something like a know a Filipino also working here or I have a lot of Filipino friends but in your case since she ask you if you understand her because she have a british accent she might thinking you don't understand her because of her accent,

On the other hand when you think its of discrimination be courteus, just act professional on the conversation it's like a slap on her face and prove that she is wrong...

very wrong...indeed

but never be rude on the phone... never especially when you are working in a company it will reflects on the whole company...

in your case you didn't act right... sorry to say
I am a Filipino too but you told her to be professional while you're in your high tone...

i think you lost your professionalism too..