It really depends on which country you want as your source.
USA: I personally use Google Voice (now integrated with Hangouts) as my tool of choice. One caveat is that it uses data (most options will) rather than your phone’s minutes, and it can be pretty data intensive if you’re using it a lot. Bonus: It’s FREE for inbound, and pretty cheap for non-US outbound.
Ireland, UK, Spain, Hong Kong, Austria, and a few others: Three Like Home offers free roaming in these countries (*at least the UK and Irish SIMs, I cannot speak to others) so it’s pretty handy if you’re Europe based.
Otherwise, I’d vote Skype In based on the price that aircall.io wanted ($30 is a bit spendy imo). ~$20 for unlimited calling to landlines globally (mobile is expensive still).