This a very common question for anyone buying a local SIM or roaming SIM card.
All phone numbers saved on your phone will stay there.
Those saved to your current SIM won't.
Download them from current SIM to your phone - or even better to a backup of your contacts in the cloud.
Settings > Mail, Contacts, Calendars > 'Import SIM contacts'
(more info here: http://support.apple.com/kb/HT4994?viewlocale=en_US )
When doing this on iOS 7 it asked me whether I wanted to save them to iCloud or Exchange - I chose iCloud as my Apple iCloud account automatically keeps a backup of these in the cloud, and it wouldn't matter if I ever lost my phone.
Go to contacts application and then tap on the Menu button.
Now you need to choose the Import/Export option (down the bottom or in settings) and then tap on the Import from SIM card.It will then ask you to choose the contacts which you want to import under your Gmail/Google Account.
Now your SIM card contacts will be saved in your Google Account i.e, it will be auto sync'd to your Google Account.
Now on your new mobile phone you just need to add your Google Account and your contacts would be automatically synced.
Bonus tip on international calling codes:
Don't forget to add the international dialling code (ie for Thai +66) to the start of your local number and drop the '0' if someone is contacting you from outside that country. eg If someone is calling your Thai number (eg. 0912345678), they must call +66912345678
In reverse - if you are calling outside Thai for example to Australia, you must add +61 to dial their Aust numbers (and drop the '0' at the start of the number also).
All international calling codes are listed here:
Got a good travel tech tip? let me know in the comments!
Written by Peter Cain
Peter Cain is the Founder and CEO of dataGO.co, a company committed to making it easier and cheaper to stay connected whilst travelling. You can find him on Twitter and Google+. You can also find dataGO on Facebook, Twitter, LinkedIn, and Google+ for all the latest info on local and roaming SIMs and data hotspots.