The cost of losing clients is probably far more than the cost of a cheap phone or laptop.
Clearly -- and therefore, throwaway83945 must be doing it for non-monetary reasons.
Why is everyone offering suggestions for roundabout ways to do exactly what OP "made the conscious decision" not to do? That's not helpful.
A regular phone works. Even a payphone, since the dial-in number is toll free, at least in the US.
I am not in the US.
All the major services have dial-in numbers in most countries. If you live in Rwanda, refuse to have a smartphone, refuse to install applications on your laptop, and refuse to use web client versions of these systems - what do you want from this thread? To be in all of these categories at the same time is either irrational stubbornness or Internet trolling.
Many of the major services also allow the conference do dial out to a regular phone number. Depending on the relationship with the client, it might be possible to just ask them to dial out to your phone. The cost is usually not that great, and while the client might think it’s odd, it should not be too much hassle.