iphone 4S cannot make outgoing calls

I have an iphone 4S running iOS 6.1.3. It has operated without problem since I bought it but recently suddenly it stopped allowing outgoing phone calls. I could receive call and receive and send sms messages but everytime I tried to make a call it would beep 3 times with the message call failed.

At first I thought it was the service provider. I am in a developing country and local services are not the best with the mobile telephone system constantly overloaded but after a week when other people around me with iphones could make calls and I couldn't.

I searched and found comments on discussions which told me of other people with the same issue. A commonly mentioned fix is to go to "date and time" and switch "set automatically" off then go to "show my caller id" and switch it off. I tried both (on my phone caller id is greyed and I cannot switch it off) without success.

Some people talked about rebooting and restoring the phone, rebooting didn't work and restoring was reported to not work so I didn't do it.

I went through the phone switching things on and off particularly leaving functions I felt I didn't need. I concentrated on things that might interfere with the phone call functions. I removed the Viber app. Suddenly the phone started working again. I am not sure what I did to fix it.

It worked well for a couple of weeks then we went into a period where everyone was having problems with the telephone service with many people not being able to make calls (on differnet types of phones). Other people's phones started to work again but mine didn't, same as before I could sms and receive calls but couldn't make calls out.

I started switching things on and off and eventually gave up. My wife had a go and suddenly the phone started working again. She was sure it was when she switched off "dial assist".

It is still working and has been for the past week.

Any ideas anyone?