With Whatsapp I would often get lag, echo and generally poor voice quality, with Signal it just works perfectly, even better than regular phone calls.
Give it a try and check if it's worth it for you.
i recall only one time where whatsapp voice was ok-ish (better then regular cellphone) quality but the lag totally killed the conversation. when someone calls me via xy-voice i usually deny it and call them back via cell. it became a meme that only cheapos do this some time ago.
international might be different as quality and lag issues are more prevalent and money is usually what keeps the conversation short.