Has anyone tried to cancel a multi-month promo plan before the period ends? Is it do-able without using a time-limited virtual credit card number? Does Tello flog you as punishment for doing that?
Did you buy the plan directly on Tello's website? (Not via Stack Social or similar?)
If so, there isn't any requirement to continue the plan: it's just a discount off the monthly rate for a specific number of months, instead of a true multi-month plan. You should be able to cancel the line directly from the dashboard via the usual Cancel Plan function, or by porting out.
The only punishment I could envision would be if Tello thinks it sees a pattern of milking their referral system for referrals-that-really-aren't, and that might show up as a loss of Tello Dollars on the referring account, or something like that. I haven't seen reports of this happening.