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some phone services do not allow a call to be dialled if a current call if active equally if there
is no cellular signal an attempt to origi nate a call will fail
chapter telephony and media control
a tbs instance can indicate whether the local hold and join functions are supported by
using the call control optional opcodes characteristic this is a bit field which a call control
client can read to determine whether it should issue these commands the values in this field
are normally static for at least the duration of a session and generally for the lifetime of the
device if the operation is successful the call state characteristic is notified informing the
clien t of the current state and callindex in the case where the opcode was originate x
this is the first time the client will be made aware of the callindex
when a write to the tbs call control point characteristic by a client fails the call control
point characteristic notifies the result of the opcode write using the following format
requested opcode
octet callindex
octet result code
octet
figure tbs call control point characteristic notification format
the resultcode indicates success or provides the reason for the failure of the operation
these codes are listed in table of the tbs specification
incoming calls inband and out of band ringtones
in traditional telephony design the first thing that a user knows about an incoming call is the
phone ringing as soon as you introduce a bluetooth connection this becomes more
complicated the phone can still audibly ring or the ring may happen in your earbuds if
youre wearing them or it can happen on both
there is a further complication the ringtone that you hear in your earbuds may be id entical
to the sound on your phone or it may be a locally generated ringing tone produced by the
earbud which will be different if its the same as the sound of your phone it requires the
presence of an audio stream from the phone which carries the s ame ringtone audio that is
being played on your phones speaker thats called an inband ringtone the problem with
an inband ringtone is that you need to set up the audio stream to carry it which may mean
tearing down an existin g audio stream to another device imagine the use case where youre
using your earbuds to listen to your tv and your phone rings for an inband ringtone your
earbuds will probably need to terminate the audio stream with the tv and replace it with an
audio stream from your phone if you accept the call thats not a problem but if you reject
it you will need to reconnect to the tv which is a poor user experience
the alternative is for the phone to notify its incoming call characteristic to your earbu ds this
contains the callindex and uri ie the uri scheme and the caller id of the incoming call
the call control client in your earbud can generate a local ringing tone alerting you to the
call if you accept the call the call control client will write an accept opcode x to the
phones call control point characteristic that instructs your earbud to terminate the audio
stream with the tv and the phone which is a different initiator will establish a bidirectional
audio stream to carry the call
section tbs and ccp
if you reject the call your audio stream with the tv remains as it has not been interrupted
your earbud will simply have mixed its locally generated ringtone with the tv audio its a
much cleaner user experience but it does mean that your earb uds will make a different ringing
sound to your phone
the use of out of band ringtones can be enhanced if the earbud can perform text tovoice
conversion allowing it to speak the phone number contained in the incoming call
characteristic as part of the c all alert if the phone supports the optional friendly name
characteristic which contains the text of the caller name from your contact list on the phone
this could also be spoken within the earbud
one final subtlety to call alerts is that the phone ca n be set into silent mode where an incoming
ringtone is not sent to the phone or pcs speaker but where the announcement is left to the
acceptor a call control client can determine this along with whether the phone supports
an inband ringtone by re ading the status flags characteristic shown in table
bit description value
inband ringtone disabled
enabled
silent mode disabled
enabled
rfu reserved for future use
table status flags characteristic for ringtone mode support
if the inband ringtone is disabled the call control client would normally announce an
incoming call at the point when it is notified that the call state characteristic has transitioned
to the incoming state note that this includes call control client s which are not acceptors
for instance a smart watch that included a call control client might vibrate or ring although
it could not accept an audio stream this highlights the fact that the call control state
machine has no immediate connection to an as e state machine one does not directly trigger
the other it is entirely up to the telephony application or operating system to link the call
control states to audio stream management
if the status flags characteristic shows that silent mode is enabled it means that the ringtone
will not be played on the phone depending on the state of the inband ringtone bit it
may be sent to the acceptor using an audio stream or as an out of band ringtone or both
if both are enabled it is up to the acceptor to decide which to use
it is worth remembering that accepting or rejecting the call along with all other state
transitions can be performed on the call control server ie the phone or pc as well as on
a call control client all connected call cont rol client s have equal access any action that
changes the state will generate a notification
chapter telephony and media control
terminat ing c alls
a call can be terminated by any call control client by a user action on the phone or by the
remote caller it can also be lost for a variety of reasons such as a fault or loss of signal on
the telephone bearer network whenever a call is terminated the call control server will use
the termination reason characteristic to notify the call control client s of the reaso n for the
termination like similar characteristics it includes the callindex to identify the call and the
termination reason as shown in figure
callindex
octet reasoncode
octet
figure termination reason characteristic
if multiple calls are terminated as might happen in the case of a network issue on a joined
call then a separate termination reason notification is sent for each callindex the
termination reasons are shown in
reasoncode reason
x incorrectly forme d uri for originating call
x the call failed
x the remote party ended the call
x the server ended the call
x line busy