I just ported our two numbers, both from 3 to Lebara and in both cases it was painless. Just pending a final bill from 3 to confirm that.
There is a way to avoid ever* having to port a number which involves
- a monthly cost currently £1.44/month (over and above your network tariff)
- a one-time new number at the outset, which you can run in parallel with your existing for as long as it takes to get all your correspondents updated
- using DATA as distinct from a call tariff
which
- will work anywhere in the world, as long as you have some internet access - so, free of any roaming charges if/when you are on eg a hotel WiFi etc.
- is not affected by (eg) the switch off of anything below 4g and the need for a VOLTE approved handset (is the case already in much of the USA)
- is, for all intents and purposes a landline number, meaning that any correspondent who uses a landline to call you isn't facing the surcharges many such providers apply to mobile calls
- is independent from any specific mobile network
and that is to rent a virtual landline number from a suitable provider (of which there are several) and give that out to your correspondents. And set your handset up to receive calls on that number.
If you want to make outgoing calls from the same "line" then there is a nominal per-minute charge. Depending on which number provider you choose, they may offer a call bundle. Or, using a different internet-based service, you can get unlimited outgoing calls for around £30 per year to any UK 01/02/03 landline, and many, many other countries inc. USA, and low per-minute rates to other destinations.
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* ever: similarly assumes the provider stays in business and/or hands their business over to a successor if not.