Unbelievable that this info is so difficult to get, so much incorrect info out there:
Just returned from US trip and this is the best solution by far:
Before leaving divert all calls UNCONDITIONALLY to voice mail: Do not do the "if not available" etc.
On Vodacom that is dialing on your phone **21*0YY14YYXXXXXXX#
YY being the 2nd and 3rd digit of your number and X the last 7 digits of your number.
So all calls get diverted to your voicemail at no charge to you and you will receive a free sms informing you of any messages left.
Good idea to update your voicemail message informing pple you are not available for a while and encouraging them to send you a whatsapp instead.
Ensure you have your voice mail PIN before leaving by calling your service provider, it's normally a menu item to reset PIN and they will sms you the new pin.
To retrieve voice mails from idiots who do not whatsapp or sms you while abroad dial +278214XXXXXXXXX (x being your last 9 digits of number) (Note the "+" - long press of 0)
When you hear your message press "1"
Then enter mailbox number +2782 14 XXXXXXXXX
Then enter your mailbox PIN
This call will cost you from most countries around R5/min
Do not do "Roaming on" etc as then you can not use your phone to dial in an emergency.
SWITCH OFF MOBILE DATA ON YOUR PHONE SETTINGS!!!
Following the above will result in no bill shock, you can still make calls/send SMS to SA or the country you are in when needed, and you can use wifi when available.
When the plane lands dial #
#002 # on your phone to cancel all diverts.
In the US I got a "T-Mobile Tourist Plan" sim card for $30 valid for 3 weeks, they have shops everywhere. This gives you unlimited local calls and international sms and 2gb fast data. After the 2gb it's unlimited at slower speed. The slower speed was still ok for Navigation and even facetime audio calls.
I took an old iphone with me, inserted the sim in this phone and created a Personal Hotspot from this phone(iphone 6 and newer has this function) (Tmobile is one of few which allow Tethering), so family members had data access all the time, received whatsapp's everywhere. Important to turn off auto updates on all phones connecting to hotspot, those phones thinks it is connected on wifi and that will go through your 2gb in no time.
Cheap travels!