Sign up with a VoIP provider that offers 911 service.
As part of the signup process, you provide a "service address" which is used to route emergency calls. This address is used to route calls regardless of where you are actually located when placing the call. (And, if E911 is supported, the address is automatically supplied to the operator.)
Depending on the provider, you may need to purchase a DID (phone number) in your home area. (~$1/mo). Usage is usually billed by the minute (~1¢/min) with optional monthly plans including minutes available.
You can access your VoIP account over a data connection (cell or wifi) using a SIP app on your phone, or many VoIP providers provide local "access numbers" that work more or less like a calling card -- you use traditional PSTN service to dial the number, then dial an account number/PIN, then dial a destination number.
As an added bonus, this means you no longer need a separate phone card -- just use your VoIP service for regular calls too.
Note that the kind of VoIP service that provides this level of flexibility is typically oriented to business customers. While very inexpensive, it can also be very difficult to configure. You may be able to find consumer-oriented VoIP plans that are easier to use, but will be far more expensive.