If you want to use "any old telephone" then you need to buy a box called an Analogue Telephony Adapter (ATA). However if you only want a single handset, it may be cheaper and more reliable to buy a dedicated SIP handset which connects over ethernet or wifi, and throw away your old analogue phone.
It would be cheaper and is often more convenient to use an app on your mobile phone. I recommend Acrobits Softphone because it uses native push notifications to wake up the app on incoming calls, so it doesn't drain your battery while waiting. It has a one-off cost which was about £6 when I last looked, or a couple of quid more for Groundwire which has some extra PABX-like features. If you have family sharing on Google Play or the Apple App Store then you only need to buy it once and all family members with the same type of device can share it.
You asked about the sequencing. The first thing you should do before anything else is to set up your VOIP service. The VOIP provider will typically give you a temporary number, which you can use to test your ATA or handset or app for both incoming and outgoing calls. Then when the number ports, you know it's all ready to go.
Next: if keeping the old phone number is very critical to you, then you should order a completely separate line from Sky, in parallel with your Vodafone one. Then when it's installed and working, you port the VOIP number, which should automatically kill the Vodafone broadband on the day of the port. This is the most reliable approach but you may end up paying for both lines for a while (e.g. Vodafone may insist on a 30 day notice period, and/or a line cease charge, if their line was not directly migrated away to another provider).
The cheaper way to do it is to do a simple migration from Vodafone to Sky using one touch switch, but getting a new number from Sky. When Sky terminates the Vodafone line, the old number goes into a sort of limbo where you have 30 days to reclaim it via your VOIP provider. This *should* work, but it's not impossible the number will get lost.
Note: have you actually spoken to Sky? Maybe they can use your 10-digit number by porting it manually. But in your position, I would in any case recommend the VOIP solution. It means you never have to worry about number migration again (unless you change VOIP provider), whatever you do with broadband.
What candlerb wrote is spot on.
If you want to keep the equivalent of a landline VOIP is the way to go and by separating your ISP from your phone service provider it is easier to change ISP.
We have two VOIP providers which are:
(1). Sipgate Basic/Starter. - (Which is no longer available to new customers).
(2). Voipify - (Which costs us £5.40 per month, (including VAT), for line rental and this includes 200 minutes per month of free 01/02 calls with a cheap cost per minute for other calls.
https://www.voipify.net/call-charges/?utm_source=goo...
Both of these VOIP Providers are OK for me.
If you decide upon a VOIP provider you will be provided with a new VOIP phone number partly chosen by you, for free, and porting of your old number is usually possible. - I have not heard of a 10 digit phone number before but if you ring Voipify they will be able to let you know if porting of a 10 digit phone number to them is possible.
Personally, I would just choose a new 11 digit phone number if you do decide upon a VOIP solution.
There is another option that may be better: As I said we have VOIP phone numbers from two VOIP providers which works well but we also have Sim Only mobile phone deals, (with unlimited calls and texts), that costs us less than £6.00 per mobile number per month.
My wife likes to have the equivalent of a landline because it provides alternative if the phone masts failed but I would probably just use the mobiles instead.