So, I was lucky enough to be able to buy another second-hand VT-10 at auction and have carried out some tests here. The command $WP+PSM=0000,3,60,1,0 activated power saving mode 3, which I had read about during my research. After 60 seconds, the quiescent current consumption dropped to approx 5mA. This behaviour was then also seen on my VT-10 with firmware version 1.025. I will now continue to test how the device behaves with position queries via SMS, etc. Many thanks again for your support.