Duncan Davidson has written a great article on choosing an iPhone for the Global traveller, however he has missed the following method which will work for any iPhone and is simple enough for most users to cope with no matter what happens when they are overseas and out of reach of their regular support network

I’ve used this method to help a few people when traveling overseas with their iPhone and/or iPad.

Most of them have kept their carrier sim, turned off data roaming, NOT set up or requested international call roaming and NOT made outgoing calls when abroad using the iPhone ‘Phone’ app. (This generally means they can continue to text normally without too much cost and there is nothing on the phone to reconfigure on the return home).

I install Skype, have them set up an online number at Skype.com and enable automatic Skype credit top ups.

I set up each email account on each device to receive AND send while overseas (many devices are set up at time of purchase to use carrier smtp settings which fail to send immediately when you are not connected to the carrier.
iCloud and gmail just work anywhere. If you own your domain and use this as your outgoing mail server where supported then this will work too – other domains/ISP emails may require the use of webmail as a fallback

I set up an email auto responder for each account which explains received messages may not be responded to for a day or two (in the event of a failure to get online) – keep in mind that this may advertise that your home is unoccupied.

Back to Skype. As mentioned the traveller purchases an online number. With this we then record a voicemail message using the Skype desktop app and then, on leaving the country, activate call forwarding from each of the travellers local phone numbers to the online number (using the method of dialling the carriers call forward activation shortcut).
The message we record might mention the email address for urgent enquiries since it will almost always be possible to respond to an email rather than a call due to the extra bandwidth required, although the traveller can always respond via text message if all else fails

When anyone at home calls the travellers normal number(s) they are redirected to Skype voicemail and are able to leave a message.
The traveller receives email notification of any message and can listen and respond when they are able to connect to WiFi (using the Skype app on an iPhone to return the call costs very little and it is easy to check your costs on a daily basis by logging in to the Skype website)

A couple of issues crop up in this scenario

1. Returned calls must be made at a reasonable hour in the callers time zone.
2. Appointments that are added to your calendar can get shifted if you have set your device to your current (out of home country) time zone.
3. If Skype is left running on any device the voice message may take a dozen rings to answer (and may even time out).
4. If the WiFi you are connected to is not good enough then call quality can be poor
5. Clients sometimes struggle to understand the difference between using Skype to make free calls to other Skype users only while they are online and making calls using ‘Skype Credit’ to the regular phone number of any person with a phone at any time of day or night
6. Siri likes to activate when you make a Skype call and raise the iPhone to your ear (turn off this feature under Settings > General > Siri > Raise to Speak)

In addition to all this you might consider carrying a Pocket WiFi device which is unlocked to be used with various sim cards in each country you travel to (in case local WiFi fails to be available or of a decent quality)