Onetel number for non-BT user

Onetel doesn't seem to advertise its tollfree number (for non-BT users) as it is dialled by the "phonepal" (sic). If I want to use my own dialler - and use Onetel only for certain calls - how do I get the right number? And might they not change it since their dialler can phone home? I note they have a clause saying you have to use Onetel as your sole provider. Has anyone heard of this being enforced?
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