Volume of SMS msgs is probably much higher than volume of VM msgs, even if they are smaller (max 160 characters each). It all multiplies out and adds up, costs money to store (on enterprise-grade HDDs) somewhere. Large corporations always have some kind of big server farm somewhere for storage and archival purposes, but I doubt that massive storage capacities are installed at every cellular node, tower, base station, or transceiver station - these "network" hardwares are too distributed, too costly to maintain and secure simply to store long-term user data, they'll be kept lean and streamlined by costs alone.
I suspect that "active" messages are kept on the network until they're delivered, but are probably flagged as "inactive" after a day or a few days or a week at most. They are then apparently archived off-network for much longer periods, many months or even years. It's worth noting there's apparently no mandated limits in wireless or consumer or privacy laws ... each operator basically does whatever it decides to do ... but there have been cases (in the USA anyways, if not in Canada) where courts have obtained transcripts of text messages from several years past.
https://www.computerworld.com/article/2471232/endpoint-security/how-long-does-your-mobile-phone-prov...
