I have a Un-common issue here, One of my customers (FreePBX 13.0.197.28). have an issue when they call out they can hear another Extention talking (only the Extension - no audio from remote Party.)
I have tested this. as below
I called from my 1001 extension to my mobile(0822222222). and i answered the call. I could then hear extension 212 talking but not the person they are talking to.
Is the other extension on the same site as the complaining user? (If they are using ATAs, it could be cross talk on the analogue side of their ATA.)
SIP is handled purely as digital and something would have to be actively done to add voice samples from one channel to another, e.g. an explicit conference bridge or ChanSpy would need to be invoked.