business use, the white list usually links to a directory service
located on an LDAP (Lightweight Directory Access Proto-
col) [5] server. For either use, however, the white list does
not usually include the addresses of persons or organizations
with weak social ties [4] such as friends of a friend in an SNS
(Social Network Service). When people who have with weak
social ties place calls for the first time, their calls are filtered
out since their caller IDs are not found in the callee’s white
list. This is called the “introduction problem.” To mitigate
this introduction problem, some systems forward these calls
to a voice mail box, rather than reject them. However, this
is not a desirable solution because it requires callee’s time to
check them and causes the delay of notifications. Thus, we
need a better approach to label incoming calls from persons
or organizations who have with weak social ties.
For this purpose, we analyze how legitimate calls from peo-
ple with weak social ties are triggered. We then propose two
mechanisms to label incoming calls by using cross-media re-
lations between calls and previous contacts. For our first
mechanism, a potential caller offers the callee his contact
addresses which he might use in future calls. If the callee
agrees, these contact addresses are added to his white list.
We describe this mechanism further in Section 4.1. For
our second mechanism, a callee provides a potential caller
with weakly-secret information that the caller can use in
future calls in order to be identified as someone the callee
has had prior contact through other means, as outlined in
Section 4.2. Section 5 describes a use case integrated with
an SNS and Section 6 describes implementation to achieve
these mechanisms. Finally, Section 7 concludes the paper.
2. RELATED WORK
Similar to preventing bulk unsolicited emails, spams, there
is no panacea for preventing unsolicited calls; thus, a col-
lection of solutions is needed. As described in [1], the so-
lution space can be divided into two categories: one places
procedural, computational, financial, and/or legal burdens
on callers, and another labels incoming calls on the callee
side. Our mechanisms using cross-media relations are used
in conjunction with adding procedural burden on the caller
side and enhancing the labeling mechanism on the callee
side. Consent-based solutions in SIP [6] are also used in
conjunction with the two categories. To grant a permission,
whereas the consent-based solutions use additional SIP mes-
sages, our mechanisms reuse messages through other means
than the SIP.
Most well-known solutions for labeling incoming calls are
based on authenticated caller IDs as described in Section 1.
Since maintaining static lists of caller IDs as white and black
lists has the introduction problem, many approaches using
social graphs have been proposed. To expand white lists us-
ing social networks, Ceglowski and Schachter [7] introduced
address book sharing with privacy as an email attachment,
while we [8] offered address book propagation within SIP
messages. To update white lists based on communication
history, Balasubramaniyan and his colleagues [9] introduced
call credentials based on the call history of a caller. Dantu
and Kolan [10] described learning systems based on unso-
licited call traffic patterns in order to update reputation and
black lists. Although their communication history limits to
calls, Shacham and Schulzrinne [11] addressed using alter-
native communication channels, web transactions to collect
potential caller IDs. This is the base work for our mecha-
nisms using cross-media relations, which we expand to use
email exchanges.
To label incoming calls without caller IDs, one of our label-
ing mechanisms is based on the destination address with sub-
addressing [12], which has already been deployed for emails.
For calls, subaddressing in the userinfo of the SIP-URI is
new, but the concept of extensions in the tel-URI is similar
to call distribution at a PBX (Private Branch Exchange).
Relying on the observation that many unsolicited calls play
prerecorded messages to decrease cost to the callers, Quit-
tek and his colleagues [13] proposed Turing tests to detect
human communication patterns. However, some legitimate
calls from government agencies, credit card companies, or
dealers among others are automated recorded messages. The
SPIT detection system proposed by Mathieu and his col-
leagues [14] relies on a SPIT characteristic that unsolicited
calls originate more error messages than legitimate calls.
However, legitimate automated calls also share this prop-
erty. Also, unless the originating carrier cooperates, it may
be difficult to measure the outgoing call volume. This infor-
mation may also be considered privacy sensitive or a business
secret.
3. LEGITIMATE CALLS FROM WEAK SO-
CIAL TIES
Our quick survey gives a rough sense of how often people
are experiencing unsolicited calls, how well-maintained con-
tact lists are effective in labeling legitimate calls, and how
legitimate calls from weak ties are initiated. In this survey,
we gathered call records of 246 calls from eight cell phones
and 136 calls from four landline phones from our colleagues
at our lab. We also asked the participants about their rela-
tionship to legitimate callers whose IDs were not found on
their contact lists.
Figure 2 indicates a significant difference in the propor-
tions of unsolicited calls between cell and landline phones.
Whereas only six percent of the incoming calls on cell phones
were unsolicited calls, 52 percent of those on landline phones
were unsolicited. We suspect that this difference was caused
by the FTC (Federal Trade Commission) regulations that
prohibit telemarketing calls to cell phones [15]. Even though
we can reduce unsolicited telemarketing calls using national
“Do Not Call”registry service [15], the effect is unfortunately
limited. This is because their jurisdiction is limited over do-
mestic telemarketers, not over international ones nor calls
using VoIP. Also, some telemarketers appear to be flouting
the law. Thus, we still need a technical mechanism to help
a callee decide whether to accept incoming calls.
Figure 2 also illustrates a difference in the proportions of
legitimate calls with known caller IDs. A larger proportion,
78 percent, of the calls for cell phones carried known caller
IDs, which were found on the contact list, compared to 18
percent for landline phones. Since people usually maintain
their contact lists on cell phones better than landlines, the
result shows how well-maintained contact lists are useful to
label incoming calls.