There are many software phones to choose between, both for Android and iPhone iOS, but the quality differs, so it pays off to try out som different applications to see which suits you best. We list some Android application that have been working well, and might be a starting point for anyone wanting to use a software phone.
The voice quality is often lower with a software SIP phone for a smartphone compared to a hardware phone connected with an ethernet cable. One reason for this is that the data transmission is quicker and more even with ethernet compared to sending radio signals to an distant antenna for mobile phones.
The ping time (the delay) is normally lower and more even for a real fiber connection compared to a mobile connection, and that gives better voice quality. Hardware phones also have better ergonomics for being used as a talking device compared to a mobile phone. A mobile phone should be possible to put in your pocket, and the ergonomics is therefore worse.
A combination of hard- and software phones works well. One phone at home for example, and then it is possible to call home for free using a mobile phone.