I’ve found an interesting attempt to give a rule on this topic: http://forum.wordreference.com/threads/je-lui-parle-je-pense-%C3%A0-lui-pronom-indirect-conjoint-disjoint.812023/?hl=fr
Of course there are counterexamples. And counter-counterexamples. And other rules mixing with this one.
The explanation is:
When you can say verb + something + to someone, you use the indirect pronoun
When you can only say verb + to someone, you use the tonic pronoun (but not if it’s verb + to something)
Parler de quelque chose à quelqu’un = Je lui parle de mon voyage
Faire attention à quelqu’un = Je fais attention à lui
Faire attention à quelque chose = J’y fais attention
Téléphoner à is one of the exceptions.
Now I don’t know if there are too many exceptions to make this a rule.
I agree with Simon’s comment : there is no rule, this is part of vocabulary learning, like for French people learning English, we need to learn weather a verb is transitive or not.
In french, verbs can be intransitive or transitive. But transitive verbs can be separated again between “transitifs directs” et “transitifs indirects”. The first ones are followed by a “complément d’objet direct”, so without the “à” préposition, and indirect transitives ones are followed by a “complément d’objet indirect”, and require a préposition (“à” or “de”).
Yeah… French is quite tricky ^^ As I heard recently, we can say that in French, every time there is a rule, there is an exception to this rule and an exception to this exception !
Here is a link for further information and exercices : http://la-conjugaison.nouvelobs.com/regles/grammaire/les-verbes-transitifs-et-intransitifs-152.php