In the US, we have active military members where they are 24/7 soldiers. There are also reservists where once a month, they get together for a weekend to do solider stuff. There's also a two week stint once a year to keep up training. So not quite as well trained as active military, but a hell of a lot more well trained than just handing rifles to someone with a slap on the backside with a "good luck" for extra measure.

Otherwise, calling a retired active military member back to active service is known as being recalled.