You will need to speak to customer services, when asked why calling then stay silent for 15 seconds and then it will take you into the menu system.
It sounds like your old network has messed up the port and the forward they have to put on their network.
Customer Services will advise how to proceed as you will have to port out the number to a new network or back to the old network.
I wouldn't assume it's the other network at fault
O2 are having plenty of porting issues at the moment
(not to mention the other creaking O2 systems!)
the other network has to put a forwarder on their HLR otherwise it wont forward the calls into o2, hence saying it sounds like the other network hasn't done so...
And its not just o2 having problems with porting, i have seen it with BT, and VM to name 2 ...
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