When you are browsing,the system holds onto a minimum of 2MB worth of airtime to enable you finish browsing. If you decide to make a call or send sms before ending these internet connections (and you have about Kshs.16,which is the cost of 2MB),then the system interprets like you do not have sufficient funds to finish browsing and at the same time make a call. Hence the call would not go through.so end the connections first before trying to make a call. But if you have more than sh16,you should be ok.
hope that helps...
From a Little Spark may burst a Mighty Flame...
Luck is when Preparation meets Opportunity. ~ Lucius Annaeus Seneca