my phone wont call one PARTICULAR number topic

Hi. I have noticed a weird problem. I can call anyone and everyone can call me. But i have this particular number which my phone wont call. It starts ringing but after about half-second it cuts and tells me that the number is not available. But the number can call me. And i also noticed that when i have both these phones next to each other, the other one turns on its display as soon as my phone dials it, but wont connect, because my phone cuts it...

What kind of problem could that be? I tried removing the contact and adding it again, as well as rebooting my phone.

P.s.: the irony is that the particular number is my gf's lol