Hello,
I have a Montana 610 mounted on my motorbikes, I use the rugged AMPS powered mounts with one hardwired to each bike for quick and easy swapping of the GPS.
I haven't seen or heard of any phone mounts that attach to them.
It may not be too difficult to custom make one though, the 4 little bolt holes that are used to hold the powered mount to the ball could be used to attach a plate, and the phone's holder attached to that plate perhaps.
It's possible the extra weight of a phone may cause stability issues, excessive vibrations etc., being held by a single mount.
My phone travels in the top box or saddle bags, but if I were to want it visible I would probably just buy a proper phone mount and mount it beside the GPS mount.