According to https://www.kamailio.org/wiki/cookbooks/5.1.x/transformations (and I'm using 5.1), I should be able to do something like `$var(num)=$(fU{s.substr,-1,10})` and `$var(num)` should be `123456789` if `$fU` is `0123456789` (only the last 10 characters are displayed). However, `$var(num)` ends up with only the last character of the substring, in this case, `9`.