I suppose such system is mandatory in a car when it's sold, such after sales changes would be fine as safety is not directly impacted.
Source: my daily drive is a car without a modem on European roads, no problem.
Be that stupid if you want to. But rest assured that insurance companies will always try to get their money back or not pay at all if something has happened. If they find out that you tampered with your eCall system and if they can somehow link some damage to that, you will be held liable.