The failings of ported numbers
The issue here highlights the downsides of porting numbers. Wherever it is possible it is worth phasing out old numbers (which some businesses will find easier than others).
There are two main reasons for this - telcos are required (certainly in the UK) to come to agreement with other telcos over creating a porting agreement within 90 days, but that regulation does not go into any financial specifics. Generally the company who's number it was originally will require that they continue to make in the region of a penny / minute from it. Move the number around over a few companies and the charges can stack up. There's only one person left holding the bill and that's the customer.
The other issue is the one you particularly ran into here - that once they've hardwired the number to it's new destination (or think they have) you are no longer their customer and they care not one whit about you. This is far more true of larger outfits where the person you initially talk to is often a peon going through a script and if you have no valid account number (etc) their systems are often designed to keep you out. (usually more to prevent phishing / fraud than for this case but the effect is the same).
Where you're able to it's worth moving numbers - most people are smart enough to look for a contacts page on a website if they number they're trying seems broken, and even the most catalogue focused companies can usually migrate within a few months, using number change announcements and a bit of brain on the website, and handled sensibly it is a pretty soft process. I'm surprised it took Planet as long as it seems to have done to offer you their own free phone numbers - they'd have been itching to do so from the outset - as their margins will be better on the non-ported numbers.