If you are entering into a legally binding contract I would very strongly suggest factoring in the costs of getting a business grade service from more than one mobile provider (as coverage varies even across a small country such as the the UK) or using other methods.
its not impossible to do this via the mobile networks but bear in mind at a festival they will be busy, and there are different priority levels for both voice and data connections, depending on how much you pay and the nature of the traffic - on most networks worldwide, calls to 112 (999 / 911) emergency services normally override every other traffic on the base station)
Consider that if one kid keels over from a dodgy pill in a crowded area where local stewards/security may not immediatelly be present, 5 people who witness this incident call 112 for Ambulance (the Emergency Services openly say they'd rather have multiple calls for the same incident rather than everyone just thinking "someone else has dealt with it"), that could result in the your audio stream being cleared to allow their emergency calls through. It might not reconnect until all the 112-calls are handled! I work on telephone systems which process 112/999 calls fairly regularly (amongst all the others) and although these usually only take 5-6 minutes max thats a very long time in broadcasting...
Another way of doing it would be to use a wireless link with guaranteed bandwidth, away from public telecoms frequencies, to get the programme audio to somewhere where a fixed broadband circuit is available. This can be more reliable but isn't cheap or effort free (you would need to deal with both specialist broadcast equipment sale/hire companies and the national Communications Ministry or their agents) however most larger broadcasters do this as they already have the kit and the frequency allocations.
Your clients may be willing to trade off a certain amount of resilience for less costs which should allow you to use the mobile networks provided you make them clear at the start of potential risks/limitations and deploy enough resources your end to monitor the output and/or quickly restart a dropped connection if that does happen.