Phone.com Business VoIP
If you want cheap, phone.com is the one for you, starting at just $10.39 per month per user (with an annual plan). The 300 minutes allocation in the plan make it about adequate for most small businesses who may prefer the low cost to some additional room for manoeuvre in terms of extra talk time. Another limitation you may need to live with is the limited CRM integrations.
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For businesses not interested in doing a detailed cots-benefit analysis of features against costs, this is a great place to start. As the business grows, and you have more breathing space, you could do a more detailed analysis and opt for a higher plan or another provider. Nothing is forever.