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My PRI (Preferred Roaming List) is also 15683 which may be the standard for XM.
XM does not offer femtocells or microcells. Even Verizon no longer offers their 4G femtocell to their consumer users except in limited circumstances. Now that almost all carriers offer WiFi Calling and almost all devices support WiFi Calling, users should use WiFi Calling instead as it does the same thing with one less device.
Femtocell = Phone to femtocell to WiFi/Router/Internet to XM/Verizon Server
Wifi Calling = Phone to WiFi/Router/Internet to XM/Verizon Server