Best CRMs for managed services

Hello,

I want to expand my B2B clients offerings to include deploying, configuring and maintenance of VOIP based telephony and SMS services to their end-user.

I was wondering whether their were any conpa ies that would allow this. Ideally I would like to configure servers on on-premises, unfortunately i'm only really seeing cloud-based services that offer comprehensive solutions.

I was looking in to Twilio for example and it looks like it could get very expensive to the point where it would provide minimum profitability to provide it to my SMB customers