I've found that the general rule of thumb is to call but be ready for them to not necessarily be redy to talk - sometiems they are, though. I'd say visiting is much more spotty, but my friend with cancer has appreciated when i've gone to see him a few times and took him something once. However, I call before I go to see if it's a good time.
Often, the best thing is relieving their burden - are there chores you can do for them or things they normally organize that you do, such as when i had a few guys over for pizza during a couple Sunday night Packer games earlier this year.
He just finished his last treatment about a week ago and has a full body scan to see if they got it all in a little over a week. I'll call him once that's over to see if he heard, but if he doesn't answer his cell phone I'll know he's probably too tired and figure he'll talk when he feels like it.
In case you're wondering about calling back (because not being certain of what's going on drives a lot of us nuts I'm sure, though prayer helps for me), because technology is fallible people will generally understand if you call and say you wanted to make sure they got the message as long as it's not right away - like 3-6 days later I'd say.
At least, that's the rule I've found with my circle of friends and family.