Occasional no audio on inbound calls

Having an issue that I just can’t get my head round.
We are getting no audio on only some inbound calls, all out bound are OK and most inbound are fine, just every now and then call comes through and no one there , I don’t think its the call as we had this happen yesterday where two separate extensions had the same issue at around the same time.
Initially I though it was a NAT issue but I am thinking if NAT was causing an issue it would be all the time.
I have made sure that everything has been updated, we are using freepbx Asterisk Ver.
Anyone got any ideas?

Do you have 10000-20000 UDP open?

OK, I have some more information on this that changes the issue.
We have now managed to speak with a few customers that are having this issue, and it would seem they are pushing ivr options and the system is either not saying anything or they are hearing invalid option (we only have 4 options) but while they are getting invalid options our phones ring, we answer and there is no one there, but the caller is still getting invalid options regardless of the option they press.
I have looked at the list of numbers and it looks like all mobile callers (although that may be a red herring as most calls are from mobiles, not as many from landline.
Any ideas?