Keeping current number and using 2 phones at the same time

Hi,
I have a Prepay O2 phone with lots of credit on it. I am thinking about switching to three mobile billpay as there is a good deal on at the moment. I intend to keep my current mobile no. When I am on the three mobile bill pay using my current mobile no, would it still be possible to use my O2 phone. I could use the O2 credit to call overseas – overseas calls are not covered by the free units under the three mobile billpay.
Thanks.

You could buy a new O2 sim and then transfer your existing credit to that. Note that your credit won't last forever, so make sure you will actually use it all.

No, you won't be able to use O2 credit while using a Three SIM. Use up the O2 credit, or transfer it to another O2 user, then move to Three.
Leo

The OP did not ask whether it was possible to use O2 credit on a Three SIM.

The question was whether the O2 credit could continue to be used, which is of course true, if a new O2 SIM was purchased (e.g. with the minimum €10 credit) and if the credit on the old O2 SIM was transferred (or "gifted") to the new O2 SIM. This new SIM could then be used for outgoing/ international calls, etc.

You're assuming the credit is transferrable! Is it?

Yes, that is the assumption. An unusual request for the phone company, but one they can technically do.