You are probably correct about them using Spinks. Looking at Fkightradar24, from time to time, I see PHI copters circling around southern Tarrant, and the northern half of Johnson County. Does resemble more of a training, or "maintenance check" flight as opposed to a patient transport mission. A lot of circling around, and sometimes the tracks appear to be that they are shooting landings, maybe 'touch & go' at the Cleburne airport.
Med 1, as I noted, told dispatch he was "returning to base" (on the transmission I was referring to), and he did end up at Med City Dallas on Forest Lane. I heard a couple of transmissions from dispatch to unit 5, but, again I did not have enough to go on to follow where he was at, or was headed. That might be Greenville, or perhaps a Corsicanna bird, that brought a patient to one of the Dallas or Fort Worth hospitals
I get the impression that PHI may be the 'default' provider for the Medical City hospitals A lot of their flights, if you look at the tracks on Flightradar, either start, or end, or at least 'touch' one of the Medical City facilities. Don't recall the exact copter, but there was one PHI bird that, when I looked it up on Flightradar, showed a picture of the helicopter with Medical City livery, though the tail number definitely was a PHI unit.
PHI, besides their own flights, also mans the Children's Hospital unit (a Sikorsky helicopter) and they also provide units for some of the Christus Healthcare missions in East Texas.