What i've seen in europe, is that you can purchase GPRS or 4G sim cards from a mobile supplier and have them linked in a kind of VPN which is tunneled to your LAN. Public IP addresses are also available, but at a cost, and more tricky to secure.
You can then schedule the interface nodes to connect periodically by a number of means, or have them online all the time. Typically this uses industrial PC's and a GPRS/G4 modem.
Actually 'calling' the remote devices i have not seen as a solution.
this is more an IT-topic rather then an PI-topic. I would recomend do talk to your IT department so that they can help you to establish a secure connection to th site. Depending on your needs this can be a connection that will be established all the time or from time to time depending on the technology used.
I would recomend that you place PINs (redundand) to the site where you want to collect data (so you can collect data independend from the connection to PI Data Server). They should be connected to the solution provided by your IT so that they can reach the PI Data Archive Server.
This is what we are doing in my company.
When there is no copper links on our remote sites or when a xDSL router is out of order.
We are using 3G router on remote site that connect automatically to GPRS network, then it creates also a VPN tunnel to connect to our headquarter.
When the VPN tunnel is up, it's transparent for you. you only have to use PI as you did before wihtout taking care of the way you connect to the remote site.
hope it helps.
What type of information are you trying to access? I'm not sure I fully understand your question.
What kind of modem are you using? Vendor, etc. Like anything that can be used as a data source, presumably your modem exposes data in certain ways (Protocols). Once we determine what those protocols are, we can try and find a PI interface to connect to the modem and retrieve the data.
At this point, any modems you are using should support SNMP. Coincidentally, we have an interface for this: PI Interface for SNMP. At the very least, you should be able to setup this interface to connect to the modem and retrieve the relevant information, whatever that may be, if it is exposed properly.