(worked in tech and telecom for 20+ years, including 8+ at BlackBerry).
In fact, we used to host the paging servers for several of the North American carriers (that's actually how Research In Motion started to get service revenue in the first place - the company initially made film editing equipment, then the paging boards for servers, eventually making the 2-way pagers which evolved into BlackBerry's).
The evolution of GSM has been TDMA, GSM, GPRS, EDGE, HSPA, HSDPA/HSUPA, UMTS, LTE. (think I didn't miss any of the steps).
There were very few CDMA carriers around the world - the vast majority went down the GSM route (and most CDMA carriers have since transitioned to UMTS and LTE.
While you may toggle between different revisions of GSM, you always have an active context on one of your carrier's base stations (be it on a tower, a repeater in a dense metropolitan area or in the subway in Boston)

The only time you don't is when your device says that there's no service.
It also used to be that you could have a voice connection, but not data. That was usually denoted by lower or upper case on your handset (upper case meant you had data connectivity).
Paging (SMS/MMS) has NEVER been a guaranteed protocol. It's always been a "fire it and forget it" - not waiting for an acknowledgement from the other side before deleting the message from the server. This is part of the reason why other message services became so popular (BBM, iMessage) - but, also because those were always included in your data package....back when SMS was usually limited to X# of messages per month - and you paid overages if you went over your limit (just like data).
As paging isn't a guaranteed protocol, Apple can't guarantee the timeliness of delivery (if they don't receive it, they can't forward it). This also means that they couldn't guarantee that there wouldn't be duplicates if the carrier was sending these to multiple devices (as either could be lost due to coverage anomalies).
True, the networks have gotten a lot more reliable over the last 20 years - so you can rely much more on paging - but it still isn't a guaranteed delivery.