It's a word I've only encountered in written form, so I'd probably trip over it and look like an idiot
[1] if I tried to say it without thinking about it properly first. Having now had time to think about it, I'd probably go for the short 'o', or perhaps "Telefónica", which is Spanish for "our-favourite-telco".
[1] You know, one of those idiots who learns new things by reading.