For phone (aka voice audio) you can have some success with 100 watts and a simple dipole antenna. You'll be better off using an antenna with more gain and/or transmitting more power, though. The highest power ham radio operators can use on HF is 1500 watts, but this requires a significant investment in amplifiers etc.

However, there are many digital modes thta can be received very far at much lower power. People are regularly received across oceans using milliwatts on https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)

The image below shows locations that were able to hear me transmitting 5 watts on WSPR from Atlanta, GA when I first got on the air couple months ago.

https://imgur.com/Z7iPzwn

I'll leave my rig receiving FT8 (https://en.wikipedia.org/wiki/WSJT_(amateur_radio_software)#...) for a while, so you'll be able to see who I can hear throughout the day by visiting https://pskreporter.info/pskmap?callsign=kn4tcf&search=Find . There's no convention on what power level to use for it, but most people keep it pretty low.