I have never done this, but this is my understanding. I think you will find useful information on www.mobileshop.org

im actually interested in the answer to this.

I had an 07976 number with orange and moved to T-mob. Wondering if a T-mob caller would pay an t-mob to orange call or same network.
And would orange user pay cross network?

I posted this in another forum which might help explain reason behind this:-

OfCOM keeps a register of which network a number was originally allocated to.  This register is not updated when someone ports numbers over (this applies to landline porting as well).

For example, having a mobile number 07000 000000 that was originally on Voda (ie OfCOM allocated that number for Voda).  If then this number is ported over to o2, BT & all other landline operators will charge as though this mobile is on Voda because they are not aware (because the register/database isn't updated) of where that number belongs to now.

Now when someone rings this number, the call is automatically routed to Vodafone and then Vodafone automatically re-route the call to the network that it was ported to (o2 in this case).  Vodafone actually then pay for the call to be routed to o2 at their own expense!

The same applies when calling from another mobile to 07000 000000 - it would still be routed to vodafone first and then forwarded on to o2.

You may not remember this, but it wasn't that long ago that o2 (PAYG anyhow - not sure about contract users) charged crossnetwork rates to mobiles that were on another network but had since ported over to o2.  For example someone on o2 calling 07000 000000 although now on o2 was charged crossnetwork rates (ie. as in calling the original network - voda).  The reason why they did this is that it cost o2 loads of money when people do this (as it does for the other network in similiar circumstances) due to the call being routed via the original network first and then back on to o2's network.

Now - you're probably thinking why isn't there a database that could be amended/updated so that all this isn't necessary?

Well, you see, that's were OfCOM (well OfTEL originally) decided it wasn't necessary because of the increase in costs in updating/maintaining this database/register.  All the networks at this time (except Orange) agreed.  Orange thought it would be better that the database/register could be amended as necessary like it is in other places in Europe (not sure about USA, etc) but this was rejected by OfTel (now OfCOM) and the other networks as too costly because at the time not that many people were porting their numbers over and therefore it was cheaper not to have a database that could be updated by each network as necessary.

As mentioned earlier similiar thing is in operation for landline portability.  For example, we were on telewest and since moved to BT (keeping number).  Now when anyone ring us, the number is automatically routed to Telewest (as our number was originally registered to Telewest) and then Telewest then forward the calls to BT with Telewest paying the cost of routing the call to BT.

Now when you port the network the number was originally on must keep a record of which network the number is on now so that it can forward it on accordingly.

Now I'm not sure what happens if you have ported your number from Voda to o2, then Orange, then finally to T-mobile.  I assume that calls are routed through each of the networks until it (finally) gets to its destination.

Obviously since the time when this was decided to be the cheapest method for the networks, things have changed.  More and more people are porting their numbers all the time.  I believe eventually though, OfCOM may agree that a updatable database be used instead so that each of the networks receiving a call could check which network the number is on now and forward it straight to them without going through all the other networks (if applicable).

I once had a friend who had an O2 phone, but with a T-Mobile number.

I had a t-mobile contract (thankfully I don't any more!!), and was able to use my inclusive minutes to call her.

Obviously the t-mobile computer saw the call as calling a t-mobile number, and charged it as inclusive minutes, as my plan allowed t-mobiles and landlines for inclusive minutes.

However the worse scenario would be, that if some networks who offer inclusive minutes to their own networks would not treat customers who have migrated to their network as inclusive minutes or treat numbers that were originally on their network as inclusive minutes.

However despite the increasing numbers of people porting their numbers over, there are in increasing number of price plans where cross network minutes apply, so it doesn't matter what network you're calling.  A few years ago, plans with cross network minutes was rare, and people took out contract mobiles to call friends and family on a certain network in the evening and weekends to avoid high BT charges.

The same can be done on "3" at anytime of the day and to any network for a far less cost (if you sign up via a mobile phone shop online that offers plenty of rebated line rental)

Legislation on Ofcom's site here.

Ofcom have a 'Number Portability Information and useful links' page here which may help.  There is also a link to a consumer-friendly Mobile Number Portability (couldn't find landline version) leaflet here.