Gatewaying to the PSTN via Matrix Application Services
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Matrix Application Services (AS) provides a way for PSTN users to interact
with Matrix via an AS acting as a gateway. Each PSTN user is represented as a
virtual user on a specific homeserver maintained by the AS. Typically the AS
is provisioned on a well-known AS-supplier HS (e.g. matrix.openmarket.com) or
is a service provisioned on the user's local HS.

In either scenario, the AS maintains virtual users of form
@.tel.e164:homeserver. These are lazily created (as per the AS spec) when
matrix users try to contact a user id of form @.tel.*:homeserver, or when the
AS needs to inject traffic into the HS on behalf of the PSTN user. The reason
for these being a visible virtual user rather than an invisible user or an
invisible sniffing AS is because they do represent real physical 3rd party
endpoints in the PSTN, and need to be able to send return messages.

Communication with an actual PSTN user happens in a normal Matrix room, which
for 1:1 matrix<->pstn contact will typically store all conversation history
with that user. On first contact, the matrix user invites the virtual user
into the room (or vice versa). In the event of switching to another AS-enabled
HS, the matrix user would kick the old AS and invite the new one. In the event
of needing loadbalancing between two SMS gateways (for instance), the user
would set visibility flags (TODO: specify per-message ACLs, or use crypto to
only sign messages so they're visible to certain other users?) to adjust which
virtual AS users could see which messages in the room.

For group chat, one or more AS virtual users may be invited to a group chat,
where-upon they will relay all the traffic in that group chat through to their
PSTN counterpart (and vice versa). This behaviour requires no additional
functionality beyond that required to support 1:1 chat.

When contacting a user, Matrix clients should check whether a given E.164
number is already mapped to a real Matrix user by querying the identity
servers (or subscribing to identity updates for a given E.164 number. TODO: ID
server subscriptions). If the E.164 number has a validated mapping in the ID
server to a Matrix ID, then this target ID should be used instead of
contacting the virtual user.

It's likely that PSTN gateway ASes will need to charge the end-user for use of
the gateway. The AS must therefore track credit per matrix ID it interacts
with, and stop gatewaying as desired once credit is exhausted. The task of
extracting credit from the end-user and adding it to the AS is not covered by
the Matrix specification.

For SMS routing, options are:
 * Terminate traffic only (from a shared shortcode originator)
 * Two-way traffic via a VMN. To save allocating huge numbers of VMNs to Matrix users, the VMN can be allocated from a pool such that each {caller,callee} tuple is unique (but the caller number will only work from that specific callee).