changeDefaultService needs clarification

[marcoscaceres]

The spec says:

The changeDefaultService() method provides a means to change the
default telephony service used by the user agent. When
invoked, the user agent runs the steps to update the default
service.

Need to make it clear that this doesn't change the system's default telephony service.

Also, I think this should set the the default telephony service for the origin, not for user agent. If I have two dialer applications running, one should not be able to change the default telephony service of the other.

[zolkis]

Granted, it is better to have an app-specific default telephony service, instead of a system-wide. In that case there would be no system-wide default telephony service.

[marcoscaceres]

I still think that there might be a system wide telephony service, which is the default one that is used when the UA is powered up - that one then propagates to each application, each of which can then switch to use a different one.

[zolkis]

IMO that does not need to be exposed in this API. It can be the choice of the implementation/platform/device. For the dialer it is enough to have a service. Actually I am also playing with the idea of removing this default service concept, and go with listing the services, and choosing one. We can also introduce restrictions, so not every dialer could handle every telephony service, but the ones e.g. the user permits.