Tello.com (US) is a recent expansion of Tello.co.uk, hence the likely reason why PAYGO US calling rate is higher than Canada's. In the US, Tello uses the Sprint network.
Another free option is wifi-calling (VoIP). If you're staying indoors for the majority of time (where wifi is presumably available), then wifi-calling can offload some of your minute/data need. A popular solution is Google Voice (GV) and Hangouts, which will need to be set up stateside, and you'll need to invest in an iOS or Android phone. Hangouts has free calling to both US and Canada, and would be ideal for your situation.
Both RingPlus (R+) and FreedomPop (FP) can be good solutions, as long as you know their caveats. Neither extends to Canada, unless you use Hangouts on top of their service. The "unreliability" cited is mainly limited to inbound voice. Outbound voice and data are as reliable as any other provider.
FP is VoIP, which relies on Sprint LTE or AT&T/TMO HSPA+ coverage. VoIP calls in a moving vehicle aren't advisable, as they may get dropped. VoIP voice quality may vary depending coverage location. (This includes Hangouts, which is also VoIP.) R+ has regular voice, but can have high latency.
As freemiums, R+/FP accounts are restricted so they can upsell. You can bypass the restrictions by using Hangouts on top of their service. For FP, you'll want to disable the bundled extra-cost options when signing up. For R+, you'll need to find an eligible Sprint phone, as Sprint has restrictions on what phones can be used.
You can get more details for R+/FP from their respective threads on here. As typical, there's lots of noise in forum threads, so suggest selective reading and skimming. Use your Google-fu, and keep up your bias filter.
As can be inferred from the recurring mentions of Hangouts above, I think it's a good complement to increase the freemiums' functionality. I'd suggest using it in place of FP's own VoIP. It's also a good wifi-calling solution, as well as a flexible tie-in to integrate the disparate accounts into one calling number.
All this info can be overwhelming, so I'll offer an action-list as a shortcut:
1. Have your US contact set up a Google Voice account, buy a cheap GSM Android phone, install Hangouts & Hangouts Dialer app on it with the GV account, and send it to you. The GV setup and Hangouts install have to be done with a US IP.
2. Drop in an existing SIM you're using. Try Hangouts for wifi-calling, and with the SIM's cell data. Hangouts uses about .7MB/minute for voice.
3. If you think VoIP via Hangouts is acceptable, get some FP GSM SIMs. Each SIM allows 600MB (usable) free monthly, so based on your 500MB/day usage, then get one SIM for each day of the trip, and swap in a different SIM daily. Use Hangouts with the SIM data to get voice/text/voicemail.
4. Use Tello or equivalent PAYGO solution as a backstop. Wifi-calling is your 1st option, FP GSM SIM your 2nd, and Tello your last. If Tello, then you'd need a 2nd phone, as Tello uses Sprint, which doesn't use GSM. Both phones would be tied to a single GV number.