I used to work with technical support, presentially, via conference call or by phone.
You could also use ...both presentially/in person OR via conference calls, landline
Calls, fax (what you have).
I just think the first option is terser than the second, it´s up to you the use of "both" here or not. But for instructional purpose I used the word, since you wanted to see it in play.
Suporte técnico fica implícito que é ao(s) cliente(s), por isso que deixei "aos clientes" fora.