Yes you do, without roaming on you will not be able to register on partnering networks
Turn on flight mode before take off, so phone isnt broadcasting, and off when you land
Most network providers charge for use of data or making calls whilst roaming unless its in your plan.
Some have used local sims from destination country or found esim travel supplier when abroad, set phone to use local sim for voice and data. Assuming you have dual sim phone you can still recieve calls from home sim but be aware of overseas charges.
Like calls going to voicemail is still chargeable call if not included in a plan
But must have roaming on from network provider, having roaming on just on your phone does nothing