Set these once so Email opens your sending Gmail and Call / Text open your Google Voice dialer — even with several Google accounts signed in. Saved in this browser, applies to all pages.
Best: the Gmail address you send from (survives sign-out/in). Or its account number — open Gmail and read the digit after /u/ in the address bar.
Open voice.google.com — the digit after /u/ in the address (e.g. /u/1/) is your dialer's number. Usually 0 or 1.