You may be able to get roaming coverage in Myanmar – this changes fast and you’ll need to ask your operator. If they can’t do it – or if, as someimtes happens, they say they can but it doesn’t work – you can buy a SIM card and top-up phone credit very cheaply within Myanmar. To buy a SIM card, you’ll need a photocopy of your passport photo page and Myanmar visa page, plus one passport-format photo to hand over.
To make calls from Myanmar to another country, dial 00 then the international code for the country you are calling, then the local area code (minus the 0). International calls can cost upwards of $5 a minute – you’re better off using WhatsApp or something similar to stay in touch.