Welcome to Vodafone Community
I have seen many posts over the years about calls failing to set up so I thought I would post a link showing the complexities involved in setting up a call on a mobile network. This link: Call Setup, shows every single step involved in setting up a GSM call (3G is slightly more complex and has a few more steps but I thought the 50 steps for GSM was complex enough), down the right hand side of the document you can see an explanation of what is going on. What I found particularly interesting was step 1 shows the user pressing the dial button on their handset, it's only when you get to step 18 that the network starts to analyse the numbers you have dialled. This entire process is happening tens of millions of times a day. This call flow shows a mobile user calling a landline, if it's another mobile being dialled there are another myriad of steps to locate, authenticate, page and connect the B end user.
Next time you get a "Call failed" or "Network busy" message have a look at the link above, if a single message in this process fails so does your call.
Happy reading, if there are any bits you want a further explanation on let me know and I will see if I can help.
I have been looking to post something like this for a while but the only stuff I had was in paper format from my HNC notes.......and I didn't particularly fancy typing it up, or sitting with powerpoint drawing it all out.