After many years of using complicated radio systems of varying degrees of complication and reliability, we have finally settled on a foolproof method for communicating callouts from top camp to base camp: mobile phones. Cheap Austrian pay-as-you-go mobiles have sufficiently good reception on the plateau for sending SMS messages, and even occasionally for conversation.
We are using the "B-Free" mobile scheme. (In 2011 we tried using another provider which picked up the T-Mobile network, however the reception was not as good as B-Free. B-Free has an annual renewal of the SIM which gets you the phone number and connection (plus some credit). More credit comes in the form of a receipt with a printed code or a card with a scratch-off secret number.
This has to be done in less than 13 months otherwise it costs a more as you essentially have to start from scratch (~EUR10 for a new SIM with EUR5 of credit). The phone cannot be used in the last month, but renewal is cheaper than starting from scratch.
The simplest way (which doesn't require understanding a number read out to you in German) is to dial *101#. This doesn't actually make a call but results in you getting an SMS-like message containing the balance details. There's no charge for this.
If you need to buy more credits for a phone the simplest way is to just ask for a "B.Free" top-up at the supermarket checkout (Unimarkt definitely did them in 2016). They'll give you a printed receipt with the code to key in. See below for how to use it.
(Other options we've used were:
You should be given a aufladecode, which may require scratching off a panel at lower right back of card.)