the only way I know to up the voltage is to put a battery in series with the extension and the telephone, normally used to boost the distance on an extension on a long line. Do this wrong you will blow the system.
I am not advising you do this, just telling you this is what was done before IP extensions took away the need for distance on tie lines,

“I have not failed. I've just found 10,000 ways that won't work.”