From what I read Carthage was able to send messengers to Hannibal as he made his "tour" of Italy. I have a few questions about the messengers themselves.
Did messengers have to disguise themselves once they were on enemy land? Did they travel alone? Once they got to Hannibal's army, how did they prove to be what they were instead of an enemy in disguise? Were there large sections of undefended Roman land that allowed a Carthaginian to simply travel through?
Hannibal controlled a large portion of southern Italy, meaning that it was no harder to communicate with Carthage than it would normally be. While he decided to march overland between Spain and Italy, most people of the time sailed, so again his connection to Spain was no worse than it would have been. The issue of course was that Rome was the superior naval power.
In general, communicating in war was difficult. There could be checkpoints where mail was read (Sidonius, writing 700 years after the Punic Wars complains of them) and Hannibal was under constant watch once Fabius Maximus took control of the campaign.