Recently, this has happened to another photographer I know who was studying in Australia and then went home. So now his Australian number does not work, and he is locked out of his account. I was lucky enough to come back to the USA whereas, unfortunately Danny is not returning to Australia so he cannot simply just wait to get his account back.
If you have 2-step notification enabled, make sure you:
1) Actually print out the back-up codes Google gives you. Those will save you if you are overseas and can't get the text message from your phone. You can get to them by going to your account settings and clicking on "download backup codes." It may ask you for your password.
2) Find out the dates that you started using all the Google products you use. It is probably the FIRST email you have in your box if you use Gmail (unless you delete them) or the FIRST in your Google Voice inbox.... etc. If you become locked out, they will ask you for the specific DAY. If you're off by a few days... no access.
3) Maybe set a backup number that belongs to your spouse / friend / mother so that if you lose access and none of the rest works.... you can still get your pin code from them. DON'T SET YOUR BACKUP NUMBER AS A GOOGLE VOICE NUMBER. If you're locked out of your account that includes your GVoice number. So if you can't get it on your cell phone (which your voice number is linked to) and you can't see the text message on the GVoice site, it does you absolutely no good to do it that way.
You could always go into your account settings before you leave and disable the feature, but if you enabled it in the first place, it was for a reason, right? That said, I personally think Google could probably have a better way of doing this service - maybe security questions that we can make up (I guarantee nobody would guess the name of my first stuffed animal and steal my identity.) HOWEVER I am not a Google Geek nor really a geek in general so there may be a very good reason why Google decided to use text messages / voice call instead of the way every financial institution seems to do it. The answer would probably contain a whole bunch of computer-y words that make my head hurt. If there is a reason why we NEED to have something we know and something we have in order to be secure, then maybe we could print out a big grid of codes - say 100 boxes with one code in each box. The computer could ask you for the code in box AA5 and then you count 27 over from the left and 5 down and that's the code you put in! That would keep people from having to rely on a phone if they are overseas.
If you are moving overseas..... just deactivate the security feature until you finish moving. Honestly it's a lot easier. Hopefully nobody will hack your account during that time. Make sure you only log on when necessary, as you will probably be on unsecured networks. Just be careful!