Ditto squared to what Lightninghorse said... the only way I know to reset the numbering plan is to do it port by port. The SX50 programs from the console using a Command/Register/Field scheme. That means there's lots of room for "fatfinger" error.

If you're not familiar with the 50 and the job needs to be done soon, I'd suggest contracting this job out to an experienced tech. Work out a deal to get some fundamental training out of the deal so you can maintain the system.

If you can't do that, take it really slowly. Get the manual, get used to the programming scheme, get a hard copy printout of the existing program, and check it carefully for any numbering conflicts before you change anything.

Good luck,

Harry at Telecom Equipment & Consulting
Specializing in Mitel systems for the Hotel/Motel industry
www.TECHarry.com