Option #1 - Convert this number to user number and add forwarder to reception number. it can be Teams client or may be number forwarded.Option #2 - Do the option#1 part of it and forward immediate Voicemail but for how many days? This wont be an option :DOption #3 - Create new Auto Attendant immediately and Add the greeting (just like other reception number) so no one feel a difference or changes. Then all the call forwarded to existing to reception number: +1 365 xxx 6226 (Not migrated reception/main line)
Best option will be Option #3 and i have created AA and added a greeting and in the call routing options I select to External phone number which will ring the existing reception number.
Unfortunately it was not working and i was checking the license etc for AA is all good. i had a Microsoft Phone System standard - Virtual User license for AA.
Fixed: It was for External call to be forwarded as redirect option from Auto Attendant it required Calling plan to be assigned but getting an error due to conflict of some license.
i assumed this was due to bundle license option and it had some license was conflicting. what i did was following and kept only domestic calling plan and assign to Auto Attendant addition Microsoft PhoneSystem standard - Virtual User license and everything works well. Try this and enjoy!