I'm afraid that you cannot do like that. When you test the dial-in conferencing, user is sure to receive the email with phone number. Because it needs to check if a user can take part in a dial -in conferencing session.
If you mean the welcome email to dial-in users, you can set the “UserEmailAddress” to your test users. So that, just certain users will receive the email.
For more information about setting welcome email, please read this article: https://learn.microsoft.com/en-us/skypeforbusiness/manage/conferencing/welcome-emails?toc=/SkypeForBusiness/toc.json&bc=/SkypeForBusiness/breadcrumb/toc.json.
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