How do i forward a voicemail
I just leave my iPhone in "airplane" mode after arriving at my destination (it is reassuring to know that this is enabled since you can easily see the airplane icon in the menubar so you know that you will not be sending/receiving any cell data). Then open up the Settings app and in the WiFi settings turn WiFi back on (but leave the Airplane mode "on") so that you can use your iPhone for WiFi only access.
Another tip is to set up a Google Voice account (google.com/voice). Configure the voicemail/text settings in Google Voice to send you an email (enable transcribing) when you get a voicemail (don't forget to record your voicemail greeting). Download the Google Voice app to your iPhone and configure for your account. The last thing I do before I board the plane leaving the USA is set my iPhone to forward all calls to my Google Voice phone number and
then I will receive an email overseas alerting me to any voicemails with a pretty good translation (at least enough to know whether or not I need to bother returning the call before coming home). You can also use the Google Voice app to text for free through a WiFi connection.
While you are using WiFi, don't forget that you can use FaceTime to connect with another FaceTime account. When you get back into the USA, don't forget to disable the phone forwarding after you disable the Airplane Mode. While your phone has forwarding enabled, you will see a phone with an arrow icon in the iPhone's menubar to remind you that it has been enabled.
I passed along an older iPhone to my daughter (who is not old enough for a phone yet) to use as a pseudo-iPod Touch and did the same thing. just enabled airplane mode and turned on WiFi.Source: apple.stackexchange.com