I'm not an expert on extensions, but it appears you've gone through all the appropriate steps. I'd keep calling and emailing everyday until they come back to you. It sped things up for me when I was waiting on a decision (ultimately got rejected) in August. I'm sure they're just reviewing it still ...
Hi All My husband and I have applied for a T2 extension (Main + dependent) from within the UK with the following timeline. It's been more than 10 weeks and we have neither received a decision nor any communication from the UKVI. We called them twice but they mentioned that they cannot tell us anythi...