Best mobile operator for roaming

Hi,

I'm going to be out of the country for a couple of years and I was wondering what would be the best mobile operator to switch to before doing so.

I would like to:

• Keep my phone number while abroad
• Have a low monthly cost (currently pay 4000/month, which wouldn't be great for holding a phone number and getting the occasional call)
• Have decent/cheap roaming (or even free with Wi-Fi Calling being a thing now)

I don't care for data roaming.

Thanks!