SPC C500 won't call... help!

[Guest SSR]

I've had my SPV C500 for around 6 months.

A couple of days back when I try and dial it shows "calling..." in the display and jumps straight to showing a connected call. But it's not connected, and then it drops back to the home page.

Other times it shows "calling..." for longer, then beeps to suggest a dropped call.

And to make things even more confusing, at other times it works fine.

I've tried pulling the battery, memory card and SIM card, and reseating. I still get the same behaviour.

I'm using the phone in Brazil, unlockied with a local SIM, but I've been using it this way for 6 months without issue until the last couple of days.

Anyone ever seen this before and/or got advice?

It would be very much appreciated as getting this repaired or replaced is going to be tricky. And this phone has been great so far...