Skip to content
Spaw
Get your API key

Calling window

published · September 27, 2026

A calling window is the span of local hours a sender contacts people in. Spaw answers in_call_window from the number's time zones: clock arithmetic, not compliance advice.

A calling window is the span of local hours in which a sender is prepared to call or text someone, such as 09:00 to 20:00 at the recipient's end. Spaw checks a window against the time zones of the number, which are the zones where it was issued and not always where the person is.

What Spaw answers

Every answer carries utc_offsets, the distinct current UTC offsets across the number's timezones, sorted west to east, and local_time, the local time now, when every one of those zones shares a single offset. Send call_window as two 24-hour local times, "09:00-20:00", with a single lookup or a batch and the answer carries in_call_window: true when the local time in every zone is inside the window, false when any is outside. The window runs from its start up to, not including, its end, may run past midnight ("21:00-08:00"), and is empty when start and end are equal, which always answers false. in_call_window is null without a window, when the number has no zone (an international network number has none), and on an answer served from the suppression list; bulk runs take no window. Nothing is stored: the clock is worked out on every request.

Most numbers have one zone, but not all. When the example mobile number of each region in libphonenumber 9.0.37 was looked up on 27 September 2026, 15 of the 244 answered more than one zone: the Russian one thirteen, the Australian one six. There local_time is null whenever the zones disagree, and a window has to hold in all of them at once. in_call_window is not one of the SMS policy's rules, so it never changes ok_to_send.

What it does not mean

The zone is where the number was issued, not where the person is: a mobile travels with its owner, a VoIP number can be answered anywhere, and a ported number keeps the zones of its code. in_call_window is clock arithmetic, not compliance advice. The US rule, for example, counts the hours at the called party's location: it bars telephone solicitation of a residential subscriber "before the hour of 8 a.m. or after 9 p.m. (local time at the called party's location)" (47 CFR 64.1200(c)(1), https://www.law.cornell.edu/cfr/text/47/64.1200, accessed 27 September 2026). No holiday calendar or do-not-call register is checked. You choose the window. Whether a country keeps one clock or several is on its page under phone facts by country.

Related

markdown version: /glossary/calling-window.md

Verify addresses the same way

The Spaw API runs every check described here on each lookup, with an SMTP handshake that never sends mail. 10 free lookups a month, no card required.

Get your API key

More glossary

  • Area-code overlay

    An area-code overlay is a new area code added over the same area as an existing one, instead of splitting it, so neighbours can hold different codes and local calls need ten digits.

  • Fictional number

    A fictional number sits in a range kept out of service for films, TV and other fiction: 555-0100 to 555-0199 in North America, and ranges Ofcom, ARCEP, ACMA and ComReg reserve elsewhere.

  • International (exit) prefix

    An international prefix is what is dialled to leave a country before the calling code: 00 in most of the world, 011 in North America. Spaw reads it and suggests a fix in did_you_mean.

  • International network number

    An international network number sits under a calling code the ITU gives a global service rather than a country, such as +870, +881, +882 or +883, so its country is null.

  • Live carrier check (HLR lookup)

    A live carrier check asks the mobile networks, at the moment of the lookup, whether a number is in service, which network serves it and whether it is roaming. On Spaw it is not enabled.

  • Number pooling (thousands-blocks)

    Number pooling splits a North American central office code into ten blocks of a thousand numbers, each assigned to a carrier separately, so one code can hold several carriers' numbers.

  • Number portability

    Number portability lets a subscriber keep a phone number when changing carrier. The number's block stays listed under the carrier it was issued to, so block data names the original holder.

  • Operating company number (OCN)

    An operating company number (OCN) is the four-character code North American numbering tables use to identify the company that holds a central office code or thousands-block.