Not sure if this is a common issue, but I think I've come across it a few times. Someone calls my VFX number from a normal PSTN line, yet they don't hear it ringing at their end. I then answer and they say "it didn't ring, you just answered" This is after the phone ringing several times at my end, so it wasn't just a case of me picking up too soon.
Send me a PM if you need some call info.

[Moderator edit: fixes spelling for better topic understanding]