For a number of reasons I decided to ditch Orange and move across to Three on a 3-2-1 PAYG plan.
I got the PAC code on Monday of this week, and sent it straight across to Three.
The number on my phone changed on Tuesday morning, and I was able to make ongoing calls on the Three sim with no problems.
But
Only calls from phones on the Three network are coming through to my phone, all other calls (from any other network) go to my old Orange voicemail.
Three's support line are very friendly, but can't give me a resolution; Orange refuse to talk to me because my account is closed.
I've sent the undivert code (? #002##) countless times using the Three sim and the deactivated Orange SIm.

Any advice on how I can progress this?

Threes porting team should be the ones you deal with, and they should be resolving the issue with Orange as it sounds as it the port hasn't completed correctly.

Your point of contact must be the Three porting team,if they aren't doing their job properly you could always email the Three Exec Office to intervene.

I don't have any advice for you, but I'm in a similar situation.
I went from Orange PAYG to an EE sim only contract, and they supposedly ported the number on Tuesday.

I can phone out, but I can't send or receive texts, and nobody can phone me because if you dial the number it says it is not recognized. Similarly the temporary number goes nowhere.
EE tell me they've had technical problems with servers, but can't tell me when or if they are going to do something about it.
Already phoned twice about it, and will phone again tomorrow.

Perhaps the server problems were at Orange's end?

There is an issue with any numbers porting in or out of Orange from 16th March. Should be resolved sometime on Friday all being well. No point phoning tomorrow as everyone is aware and the port will clear in time and a phone call won't speed it up.

True, but it would allow people to register their concerns and allow Orange/EE the opportunity to help with any problems caused by this. But that would be good customer care so is unlikely to happen.

Thank you for that information.
If only the Three help desk had known this, and Orange (EE) weren't so obstructive then I wouldn't have been so concerned.
I will sit and wait.

Well I did phone today, and was told that they were having to port the numbers manually, and the girl didn't know when it would be fixed.
I emphasised that a mobile phone that nobody could call me on was completely useless, but all she said was that she couldn't do anything about it.

I'll give it until Tuesday, but then I'll have to insist that they do something to resolve it, even if it involves giving me a different working number.

They seem completely incompetent.
If you look on the EE support forum there is thread upon thread of people who have porting problem with EE, and they sometimes don't get resolved for weeks.
It seems if the port goes through first time then it's OK, but if it doesn't they haven't really a clue how to fix it.

I'm glad my Three to EE number port went quite smoothly had no idea people were struggling with this hopefully it gets resolved soon for people.

Resolutions
The number was fully ported over sometime this morning - and I've been getting the texts I missed ever since.
I also got an email from Three this afternoon apologising for the delay, blaming Orange, and saying they didn't know when it would be fixed.

I'm just glad I don't rely on my phone as part of my job.

Donor-Led porting such as used in the UK is more prone to these issues. That's why only the UK and India uses it.

Yeah mine's working properly as of this morning as well, and I can send and receive calls and texts on my ported number at last. Only took a full week since I requested the port.

Couple of small niggles.
When you phone the number there's a delay of about 4 seconds before it starts to ring.
And when I go to Settings>About Phone it still says my number is the temporary one.
I can live with that though.

I ported my number from o2 to 3 a couple of years ago and even though my S5 is the second phone I've had with 3 it still lists a different number in Settings!