In reply to Jasonic:
There is a plan. It's only part-way through happening but it is in the process of improving the mobile reception across the Highlands. Most of the new stuff is 4G. Many of the improvements completed so far are nearer Inverness. Predictable in a way.
In the meantime, many of the basics remain the same.
Vodafone generally has the best coverage of mountainous parts of the Highlands. O2 is similar but is a lower powered network. Both are still mainly 900 MHz 2G in rural Highland areas (a few have EDGE which improves data rates). If you want use data on 2G then use Opera Mini for your browser (the Opera server chops up the website into just the essentials).
EE usually has 3G on every mast. The 3G is 2100 MHz and even their 2G is 1800 MHz. The masts are too far apart and the mountains totally mess it up.
I'd say it's between Voda and EE. You are more likely to get Vodafone but if you get EE it will probably have a higher data rate.
In mountain emergencies, 999 roaming allows you to make emergency calls on any network. If making a 999 call with dodgy reception, make certain that you plan the call to get the essentials across to the police effectively and reasonably quickly. With 999 roaming, you cannot be called back. Your battery energy needs careful management. Arrange to call again in maybe 30 minutes: getting their incident number will help.
(Most important of all, do not go to Glen Affric or Knoydart for four days and tell your loved ones that you'll ring home every day. Not gonna work. Your loved ones won't get a call but twenty other folks will!)
Post edited at 01:24