Please provide us with the dates/times and mobile numbers that have been affected by this by emailing the details over to our Support Helpdesk. 

This issue is likely to be due to call forwarding that may be set up on a phone extension line which is part of a ring group on a main number.  

So if an incoming call is received on the main number and it is set to ring that particular extension number - it would call that forwarding number as well (that the extension has been forwarded to.

We can overcome this by turning off 'ignore call forward' if a group ring.  

So this way if a divert is set up on an extension line that is part of a group ring - then it will not call out to the diverted number when a call comes into the main number – but if someone then ‘transfers’ the call to the extension then it will call out to the diverted number.