We assume that you already have Teams subscription with Phone System enabled. The cheapest is E1 + phone system add-on for $16/month
Calling plan add-on license is $12/month for Domestic (includes 3,000 minutes does not roll over)
or $24/month for Domestic + International ((includes 3,000 domestic minutes + 600 international,does not roll over)
or Usage based calling plan with auto-debit from your credit card on file. Official price list.