On number transfer and SIM keeping, if the phone is purely 3G then it is at least 6 years old, because I am pretty sure every phone since 2014 would have had at least LTE if not 4G capability. A phone of that age could possibly need a new SIM today, but that is no big deal, your existing phone number will be kept, just allocated to a new SIM. For this reason ‘backing up your contacts’ to the card is potentially not viable, sure if it’s just the physical size of the SIM then they can be ‘cut down’ to suit the new phone, but if that surgery goes wrong then it’s kaput.
JAP’s suggestion of hand recording and re entering of required numbers is likely the most ‘reliable’ method, though certain to be tedious.
On network access, this sometimes comes down to the phone as much as the network.
By virtue of our phone update cycles I am usually half a year ahead of my wife in phone update, then she updates hers and we both have the same until I update again. We are generally a year or two in advance of my parents.
I am with Optus and so is my wife, my parents are with Telstra, sometimes I have signal when my wife doesn’t, sometimes I have signal when my parents don’t, but rarely do any of them have signal when I don’t.
I recently flew Adelaide to Sydney and about halfway there are 37 or 39 thousand feet or whatever was surprised to realise that I had not put my phone into flight mode (it’s not quite the crisis you might think it is) and astounded to note that I had a single bar of reception which I think is a detail Optus best not advertise.
Lastly, and this doesn’t concern everyone, but I can fly to Europe (including middle eastern stop overs), NZ, Indonesia, Thailand, Malaysia, Singapore, Philippines, and probably any other country whilst continuing to use my phone as I do in Australia (ie heavily) and return home to a bill $10 higher than if I had not left.
When my parents travelled to Hawaii year before last, despite my advice to get a seppo sim they just used their Telstra service mobiles and returned home to a bill $800 higher, for only ten days. Now credit to Telstra, I did manage to convince them to decide to waive the $800, and whilst it’s apparent I do enjoy a good argument even for $800 I don’t really think it was worth the effort, going with optus and paying the $10 if required is far easier.