if the verb has "verbe transitif indirect" and the preposition is à could i use lui or leur.
Correct.

But these are not the only indirect object pronouns you could use, because
me, te, nous, and
vous can also be indirect object pronouns. Keep reading...
In French, we "telephone
to someone" (we do not "telephone someone"). You can tell this from the dictionary, because when you look up
téléphoner and read through the entry, it says:
Empl. trans. indir.
a) Téléphoner à qqn. Se mettre en communication téléphonique avec quelqu'un.
The person who receives the call is the indirect object as indicated by the preposition
à, so you must use an indirect object pronoun if you want to use a pronoun at all. "Lui" and "leur" are both indirect object pronouns (so are
me, te, nous, and
vous).
I telephoned "to" Jenny. >>
J'ai téléphone à Jenny.
I telephoned "to" her. >>
Je lui ai téléphoné.
I telephoned "to" Jenny and John >>
J'ai téléphone à Jenny et à John.
I telephoned "to" them
>> Je leur ai téléphoné.
Jenny telephoned "to" me. >>
Jenny m'a téléphoné.
Jenny telephoned "to" you all >>
Jenny vous a téléphoné.
etc.
sometimes a verb can not use a preposition and mean one thing but have one and mean another.
This is also true. A good number of verbs have intransitive and transitive forms both. For example, "to work" -
travailler. You will find entries for both forms:
v.i. travailler = to work, to exercise a professional activity.
He works in a factory.
Il travaille dans une usine.
His wife works too.
Sa femme travaille elle aussi.
Don't work too much!
Ne travaille pas trop !
v.tr. direct travailler [+quelque chose] = to fashion, to shape, to cultivate, to improve [+something]
She kneads/works the bread dough.
Elle travaille la pâte.
He practices/improves his English skills.
Il travaille son anglais.
v.tr. indirect - travailler à [+projet, cause, ou infinitif] = to work on/towards, to devote effort to
He is working on (working to finish) an article.
Il travaille à un article
She is working on perfecting her English.
Elle travaille à perfectionner son anglais.
Does that help at all?