New Exchange Numbering Cutover Failure
A new digital local exchange with 2,000-line capacity is connected to an existing PSTN by a single 2 Mbit/s trunk. The parent switch uses five-digit local numbers beginning 2 through 7. The new exchange has been allocated the range 81000-82999. During commissioning, subscribers on the parent switch can call 81xxx successfully, but subscribers on the new exchange cannot call 82xxx numbers. The new exchange currently strips the leading 8 and sends only the last four digits (1xxx or 2xxx) to the parent, whose translation table has an entry for 81 only. Calls from the new exchange to parent local numbers work. The numbering plan reserves 80 and 89 for service codes. Cutover is in 48 hours, 1,200 lines are ready, and the operator wants no change that could affect existing 81xxx customers.
1. Why do 82xxx calls from the new exchange fail while 81xxx calls from the parent succeed? Trace the digits.
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The parent can call 81xxx because its translation table has a route for 81 to the new exchange; the new exchange receives the full 81xxx and rings the subscriber. In the reverse direction the new exchange strips the leading 8 and sends only 1xxx or 2xxx. The parent receives a four-digit number that does not match the 82 route and may even match an existing 2xxx-7xxx local number or be invalid. Also, even if the new exchange sent 82xxx, the parent has no translation entry for 82, so the call would still fail. The reserved 80 and 89 codes are not the immediate cause, but they show the plan has service-code constraints. The fault is digit manipulation plus a missing route, not a trunk failure, because calls in the other direction work.
2. Propose the least-risk change to the new exchange and parent switch, and state what must not be changed.
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On the new exchange, stop stripping the leading 8 for calls to the parent and send the full five-digit 82xxx. On the parent, add a translation/route entry for 82 that points to the new exchange trunk; keep the existing 81 entry unchanged. Do not renumber the 81xxx customers or touch the 80/89 service-code translations. Check that 82 does not overlap any existing 2-7 range and that the trunk has enough circuits for 1,200 lines at the expected busy-hour erlang. If the parent uses a prefix-based route, the entry should match 82 and pass the last five digits; if it uses a full translation table, add 82xxx to the new trunk group. Apply changes in a maintenance window with a rollback copy of both translations.
3. Give a cutover test plan that proves both directions and protects existing customers.
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Before cutover, test from a parent line to 81xxx and 82xxx; from the new exchange, call 81xxx, 82xxx, a parent 2xxx-7xxx number, and an external number. Verify calling line identity, ringing, answer, and release. Run 10-20 simultaneous calls on the trunk to check busy-hour load and blocking. Confirm the 80/89 service codes still behave correctly. After each translation change, repeat the matrix and compare against the pre-change baseline for 81xxx. Keep the old translation loaded for rollback, monitor trunk utilisation and call-failure counters for 24 hours, and have a back-out procedure if the 81xxx success rate drops.