I would suggest Direct Routing as the best option in this scenario. If your users have M365 E3 then the phone system add-on will be required (phone system is included in E5 but add-on required for E1/E3). You will also require an MS-Teams certified SBC but the majority of sip trunk providers will offer a 'carrier-hosted' service whereby they host the SBC and provide a subdomain which will need to be added as a domain in your 365 tenant.
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Ayman_Fathy (New Contributor)
Thanks for your reply. I just need to make sure that using Direct routing with Skype for Business 2015 on-primes is doable or its applicable to co-exist direct routing with hybrid voice in my scenario.