Have you spoke to Vodafone - are there service issues where you are travelling to/from. Have you got location services turned on - And under System Services - Cell network search On - if close to or crossing international time lines you may get better service by turning the Setting time zone service off.
Also, unless you need it on for say an attached keyboard, conserve power by disabling Bluetooth.
Keeping wifi on is said to improve signalling awareness when on Data Carrier - but as yet I have noticed no difference - so again you can try turning this off when not on a wifi network.
Further power and signal reception issue could also be allayed by turning your mail retrieval service to manual\fetch in Settings\Mail,Contacts,Calendars\Fetch by turning Push Off - so mail only downloads when you go to your mailboxes.
Set Brightness to manual rather than auto and have it set at least halfway or above thats necessary for your eye comfort. All are worth a try. keeping browsers and net services to a minimum when not in use (tap home button twice and then press on any App icon till it wiggles and close it) - will also conserve more power for signalling work.
Conservation of network services is a default for Airplane Mode - the fact that this gives you better service at times seems to indicate that too much network service is running in the background. So limiting icloud services to a minimum may also help you. Finally unless you really need it on - turn Ping OFF, turn Facetime Off - both are bandwidth hogs.
regards Ct