Spending 10 minutes on the phone to answer questions is one thing, as long as they are respectful, but more than that means that they want you to work for them.
Theoretically maybe. In practice, no one to a first approximation is going to have an issue with you spending an hour helping a prior employer with some cleanup related to your prior job. Employment agreements tend to use words like outside employment/consulting/etc. They don't (and it would probably be unenforceable if they did) generally forbid you from talking to a former employer.
In some circumstances, it may even be violating your termination agreement with the former one.
Often, if made redundant by a company, you cannot return to work for them (as employee or contractor) for a year, otherwise you have to pay back your redundancy pay.
That leads to a potentially hilarious conversation about rates.
"I'll do it, my rates are 100 per hour plus 10K if you want me to start before June next year"
If that's a contractual obligation between them and you they just need to waive it in writing so that you're protected.
That being said, the issue with redundancy packages is that taxes are involved and those guys will be inflexible if they find out that your package turns out not to be tax-free after all.
Spending 10 minutes on the phone to answer questions is one thing, as long as they are respectful, but more than that means that they want you to work for them.