If I install 20meters of ethernet unshielded utp wire (rj-45) together with a phone twisted pair wire (rj-11), one touching the other, will create interferences between them?
No, it is extremely unlikely. This is done everywhere. It is very common to use the "unused" pairs in typical 10-Base T ethernet connections for either traditional analog or even digital phone service. The construction of "category" ethernet cable has precision twisting of the four pairs exactly to minimize any crosstalk between the ethernet and the telephone signals.
Shouldn't be a problem generally speaking. The ethernet carries digital signals on the wires and the two-pair POTS wires carry analog signals. If crosstalk or other interference did occur I think it would most likely be during ringing of the phone, when current was being drawn through the wire to ring the phone(s).