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Two phones.

(or a double SIM phone, but then you still risk messing up yourself by mixing data / calling from the wrong number / etc)

Try anveo.com, they have numbers globally which start at less than $1 per month but with a setup fee of a couple of dollars and a per-minute fee of around 1 cent per minute. You can also pay more per month charges for no per minute charges.

You'll also need a SIP client. Some Android phones have these built in but you can also use things like Grandstream Wave. I have no idea about iOS, MAc aor Win but there should be several options for those.

I have our work VOIP account set up on my Android phone. It's integrated into the dialer and works for incoming and outgoing calls. On outgoing calls I have to select whether I want to use the SIM or the VOIP account.

It's a geographic (local) number to our office and costs £7.99 per month. Seems like it would be easy enough to add one for personal use as well.

Two phones.