Aside from the VHF Freqs they use in the Lowell Division, they typically operated on a few different LTR Trunked Systems. I used to hear the Haverhill Division Units on the
Dispatch Communications LLC LTR System Georgetown Site. I don't typically monitor this system, so I cannot confirm any recent activity. The NH ALS units were dispatched on the LTR and then signed on with the respective fire departments prior to the loss of these contracts. The Haverhill 911 trucks are dispatched by HFD on their primary frequency, I'm not sure how they work the Boxford contract, but I believe they split the town with another private EMS agency. They did briefly run their own 2 freq LTR system for the Lowell area, but I don't think they use that anymore. Interestingly, you can still hear their operations on 453.6125 CSQ.
They had been using the old Patriot Freq within the past month to relay their calls to their dispatch. The Marlborough and Hudson units are traditionally dispatched by their fire departments.