The 7818 is an 18V regulator - operating with an input voltage of 12V clearly it's not going to function well - downstream of that you'll suffer the forward voltage drop across the reg.

JohnT, I'd use the Sprint Min/Max pot settings to get full range - think that's where your thoughts are, too.

Personally I'd probably remove the HV regulator and just connect pins 1 and 3 together (Vin to Vout) and rely on the Sprint's internal regulated supply - it'd almost certainly be fine, but.... min/max pot is fine.