Tromboning is the method used by some operators providing access to mobile services in the UK. It is cheaper to route the calls via the USA and back into the UK than to do it directly to a mobile. The downside is longer call set up times and sometimes poorer call quality.
I have just invented a new type of tromboning. My friend Terry has an American daughter. She is over on holiday (vacation) and he got her a pay as you go SIM from Orange (Orange). There was as usual a bewildering choice of tariffs. He settled on one that charged 20p per minute for any UK network including landlines and only 6p for calls to the USA.
If she needs picking up from town she calls he mum (mom) in Oklahoma and mum calls dad in Lincoln (England) using Skype (insecure consumer VoIP). Even if she used her landline service it would still be cheaper than a call to a UK network from her UK pay as you go mobile.