Dial Plans
The dial plans implemented by local telephone operating companies vary depending on whether an area has overlays, which are multiple NPA codes serving the same area, and whether the jurisdiction requires toll alerting, expressed by a leading 1 for regional toll calls. The NANPA publishes dial plan information in its information for individual area codes.
The standard dialing plans in most cases are as follows:
| Local within area code | Local outside area code | Toll within area code | Toll outside area code | |
|---|---|---|---|---|
| Single code area, with toll alerting | 7D | 7D or 10D | 1+10D | 1+10D |
| Single code area, without toll alerting | 7D | 1+10D | 7D or 1+10D | 1+10D |
| Overlaid area, with toll alerting | 10D | 10D | 1+10D | 1+10D |
| Overlaid area, without toll alerting | 10D or 1+10D | 1+10D | 10D or 1+10D | 1+10D |
Most areas allow permissive dialing of 10D or 1+10D even for calls that could be dialed as 7D. The number of digits dialed is unrelated to whether a call is local or toll when there is no toll alerting. Allowing 7D local dialing across an area code boundary (which is uncommon today and only possible with toll alerting) requires NXX protection on the other side to avoid dialing conflicts.
Most areas permit local calls to be dialed as 1+10D except for Texas, Georgia, and some jurisdictions in Canada which require that landline callers know which numbers are local and which are toll, dialing 10D for all local calls and 1+10D for all toll calls.
In almost all cases, operator-assisted calls require dialing 0+10D.
Read more about this topic: Telephone Numbers In The United States
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